Filed Pursuant to Rule 424(b)(5)
Registration No. 333-265085

Prospectus Supplement to Prospectus dated May 31, 2022

29,882,000 Shares of Common Stock

We are offering 29,882,000 shares of our common stock, par value $0.001 per share. Our common stock is listed on the Nasdaq Global Select Market under the symbol “ALLK.” The last reported sale price of our common stock on September 16, 2022, was $4.58 per share.

We are a “Smaller Reporting Company” as defined under the federal securities laws and, as such, we have elected to comply with certain reduced public company reporting requirements for this prospectus supplement and may elect to do so for future filings. See “Prospectus supplement summary—Implications of being a Smaller Reporting Company.”

Investing in our common stock involves risk. Please read the information contained in and incorporated by reference under the heading “Risk factors” beginning on page S-8 of this prospectus supplement and under similar headings in other documents that are filed after the date hereof and incorporated by reference into this prospectus supplement and the accompanying prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

The underwriters expect to deliver the shares of common stock to purchasers on or about September 21, 2022.

The date of this prospectus supplement is September 19, 2022

Table of contents

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ABOUT THIS PROSPECTUS SUPPLEMENT

This prospectus supplement and the accompanying prospectus are part of a registration statement on Form S-3 (File No. 333-265085) that we filed with the Securities and Exchange Commission, which we refer to as the “SEC,” utilizing a “shelf” registration process.

We are providing information to you about this offering of our common stock in two parts. The first part is this prospectus supplement, which provides you with specific information regarding the terms of this offering and our common stock, and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus. The second part is the accompanying prospectus, which provides more general information, some of which does not apply to this offering of our common stock. Generally, when we refer to this prospectus, we are referring to both parts of this document combined.

Before buying any of the common stock that we are offering, we urge you to carefully read this prospectus supplement and all of the information incorporated by reference herein, as well as the additional information described under the headings “Where you can find more information” and “Incorporation by reference.” These documents contain important information that you should consider when making your investment decision.

To the extent there is a conflict between the information contained in this prospectus supplement, on the one hand, and the information contained in any document incorporated by reference in this prospectus supplement that was filed with the SEC before the date of this prospectus supplement, on the other hand, you should rely on the information in this prospectus supplement. If any statement in one of these documents is inconsistent with a statement in another document having a later date—for example, a document incorporated by reference in this prospectus supplement—the statement in the document having the later date modifies or supersedes the earlier statement. We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference herein or therein were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

Neither we nor the underwriters have authorized anyone to provide information different from that contained or incorporated by reference in this prospectus supplement and the accompanying prospectus, including any free writing prospectus that we have authorized for use in this offering. When you make a decision about whether to invest in our common stock, you should not rely upon any information other than the information contained or incorporated by reference in this prospectus supplement or the accompanying prospectus, including any free writing prospectus that we have authorized for use in this offering. Neither the delivery of this prospectus supplement or the accompanying prospectus, including any free writing prospectus that we have authorized for use in this offering, nor the sale of our common stock means that information contained or incorporated by reference in this prospectus supplement and the accompanying prospectus, including any free writing prospectus that we have authorized for use in this offering, is correct after their respective dates. It is important for you to read and consider all information contained in this prospectus supplement and the accompanying prospectus, including the information incorporated by reference into this prospectus supplement and the accompanying prospectus, and any free writing prospectus that we have authorized for use in connection with this offering in making your investment decision.

We are offering to sell, and seeking offers to buy, our common stock only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the accompanying prospectus and the offering of the common stock in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement and the accompanying prospectus must inform

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themselves about, and observe any restrictions relating to, the offering of the common stock and the distribution of this prospectus supplement and the accompanying prospectus outside the United States.

This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

Unless otherwise stated, all references in this prospectus to “we,” “us,” “our,” “Allakos,” the “Company” and similar designations refer to Allakos Inc. This prospectus supplement, the accompanying prospectus and the information incorporated by reference herein and therein contain trademarks, service marks and trade names of Allakos Inc., including our name and logo. Other trademarks, service marks and trade names referred to in this prospectus supplement or the accompanying prospectus or the information incorporated by reference herein and therein are the property of their respective owners.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement and the accompanying prospectus may contain, the documents incorporated by reference herein may contain and management may make certain “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical facts, may be forward-looking statements. Words such as “anticipate,” “believe,” “could,” “estimate,” “expects,” “intend,” “may,” “plan,” “potential,” “predict,” “project,” “should,” “will,” “would” or the negative of those terms, and similar expressions that convey uncertainty of future events or outcomes.

Forward-looking statements include, but are not limited to, statements about:

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Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements that we make. These forward-looking statements involve risks and uncertainties that could cause our actual results to differ materially from those in the forward-looking statements, including, without limitation, the risks set forth under the heading “Risk Factors” in our most recent Annual Report on Form 10-K, our most recent Quarterly Report on Form 10-Q, this prospectus supplement, the accompanying prospectus and in our other filings with the SEC. Moreover, we operate in a very competitive and rapidly changing environment. New risks emerge from time to time. It is not possible to predict all risks, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements we may make. Given these uncertainties, you should not place undue reliance on these forward-looking statements. We qualify all of the forward-looking statements in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein by these cautionary statements. Our forward-looking statements speak only as of the date of this prospectus supplement. Except as required by law, we assume no obligation to update these forward-looking statements publicly, or to update the reasons actual results could differ materially from those anticipated in any forward-looking statements, whether as a result of new information, future events or otherwise.

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Risk factors

Investing in our securities involves a high degree of risk. Before making a decision to invest in our securities, you should consider carefully the risks and uncertainties described under the heading “Risk Factors” contained or incorporated by reference in this prospectus supplement and the accompanying prospectus, including the risk factors incorporated by reference herein from our Annual Report on Form 10-K for the year ended December 31, 2021, and our Quarterly Report on Form 10-Q for the quarter ended June 30, 2022, as may be updated by our subsequent annual reports, quarterly reports and other filings we make with the SEC. The risks described in these documents are not the only ones we face. There may be other unknown or unpredictable economic, business, competitive, regulatory or other factors that could harm our future results. Past financial performance may not be a reliable indicator of future performance, and historical trends should not be used to anticipate results or trends in future periods. If any of these risks actually occurs, our business, financial condition, results of operations or cash flow could be harmed. This could cause the trading price of our common stock to decline, resulting in a loss of all or part of your investment.

Risks Related to this Offering

We have broad discretion in the use of the net proceeds from this offering and investments and may not use them effectively.

Our management will have broad discretion in the application of the net proceeds from this offering, including for any of the purposes described in the section titled “Use of Proceeds,” and you will be relying on the judgment of our management regarding the application of these proceeds. You will not have the opportunity, as part of your investment decision, to assess whether the net proceeds are being used effectively. Our management might not apply the net proceeds in ways that ultimately increase or maintain the value of your investment. If we do not invest or apply the net proceeds from this offering in ways that enhance stockholder value, we may fail to achieve expected financial results, which could cause our stock price to decline.

Investors in this offering will pay a substantially higher price than the book value of our common stock and therefore you will incur immediate and substantial dilution of your investment.

The public offering price of our common stock in this offering will be substantially higher than the net tangible book value per common share based on the total value of our tangible assets less our total liabilities immediately following this offering. After giving effect to the sale of 29,882,000 shares of our common stock in this offering at the public offering price of $5.02 per share and after deducting underwriting discounts and commissions and estimated aggregate offering expenses payable by us, you would experience immediate and substantial dilution of approximately $0.73 per share, representing the difference between our adjusted net tangible book value per share as of June 30, 2022 after giving effect to this offering and the public offering price. For a further description of the dilution that you will experience immediately after this offering, see the section titled “Dilution.”

Raising additional capital may restrict our operations or require us to relinquish rights to our technologies or product candidates, and if we sell shares of our common stock in future financings, stockholders may experience immediate dilution and, as a result, our stock price may decline.

Until such time, if ever, as we can generate substantial product revenues, we expect to finance our cash needs through a combination of equity offerings, debt financings, partnerships and marketing, distribution or licensing arrangements. We do not have any committed external source of funds. We may also from time to time issue additional shares of common stock at a discount from the then current trading price of our common stock. As a result, our stockholders would experience immediate dilution upon the purchase of any shares of our common stock sold at such discount. In addition, as opportunities present themselves, we may enter into financing or similar arrangements in the future, including the issuance of debt securities, preferred stock or common stock. If we issue common stock or securities convertible into common stock, our stockholders would

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experience additional dilution and, as a result, our stock price may decline. Debt financing, if available, may involve agreements that include covenants limiting or restricting our ability to take specific actions, such as incurring additional debt, making capital expenditures or declaring dividends.

If we raise additional funds through partnerships or marketing, distribution or licensing arrangements with third parties, we may have to relinquish valuable rights to our technologies, future revenue streams or product candidates or to grant licenses on terms that may not be favorable to us. If we are unable to raise additional funds through equity or debt financings when needed, we may be required to delay, limit, reduce or terminate our product development or future commercialization efforts or grant rights to develop and market product candidates that we would otherwise prefer to develop and market ourselves.

Sales of a substantial number of shares of our common stock in the public market could cause our stock price to fall.

If our existing stockholders sell, or indicate an intention to sell, substantial amounts of our common stock in the public market, the trading price of our common stock could decline. Further, the holders of a majority of the shares of our common stock are entitled to rights with respect to the registration of their shares under the Securities Act. Registration of these shares under the Securities Act would result in the shares becoming freely tradable without restriction under the Securities Act, except for shares purchased by affiliates. Any sales of securities by these stockholders could have a material adverse effect on the trading price of our common stock.

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Use of proceeds

We estimate the net proceeds to us from the sale of 29,882,000 shares of common stock in this offering will be approximately $140.5 million, after deducting the underwriting discounts and commissions and estimated offering expenses payable by us.

We currently intend to use the net proceeds from this offering, together with our existing cash, cash equivalents and marketable securities, for the preclinical and clinical and other work associated with advancing the development of lirentelimab (AK002) and AK006 and for working capital and other general corporate purposes.

Pending the specific use of net proceeds as described in this prospectus supplement, we intend to invest the net proceeds to us from this offering in short and intermediate-term investment grade instruments, certificates of deposit or guaranteed obligations of the U.S. government in accordance with our investment policy.

Our expected use of proceeds from this offering represents our current intentions based on our present plans and business condition. As of the date of this prospectus supplement, we cannot predict with certainty all of the particular uses for the proceeds to be received upon the completion of this offering or the actual amounts that we will spend on the uses set forth above. We may also use a portion of the proceeds to in-license, acquire or invest in additional businesses, technologies, products or assets. Although we have no specific agreements, commitments or understandings with respect to any in-licensing activity or acquisition, we evaluate these opportunities and engage in related discussions with other companies from time to time.

The net proceeds from this offering, together with our existing cash, cash equivalents and marketable securities, will not be sufficient for us to fund our clinical programs, and we expect to need to raise additional capital to achieve our business objectives.

The amount and timing of our actual expenditures will depend on numerous factors, including the results of our research and development efforts, the timing and outcome of any ongoing or future preclinical studies and clinical trials the timing and outcome of regulatory submissions and any unforeseen cash needs. As a result, our management will have broad discretion over the use of the proceeds from this offering.

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Dividend policy

We have not declared or paid any cash dividends on our capital stock since our inception. We intend to retain future earnings, if any, to finance the operation and expansion of our business and do not anticipate paying any cash dividends in the foreseeable future. Payment of future cash dividends, if any, will be at the discretion of our board of directors after taking into account various factors, including our financial condition, operating results, current and anticipated cash needs, the requirements and contractual restrictions of then-existing debt instruments and other factors that our board of directors deems relevant.

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Dilution

If you invest in our common stock in this offering, your ownership interest will be diluted immediately to the extent of the difference between the public offering price of our common stock and the as adjusted net tangible book value per share of our common stock immediately after this offering.

Our historical net tangible book value as of June 30, 2022, was approximately $222.8 million, or approximately $4.06 per share of common stock based upon 54,844,080 shares outstanding as of June 30, 2022. Historical net tangible book value per share represents the amount of our total tangible assets less total liabilities, divided by the number of shares of our common stock outstanding as of June 30, 2022. After giving effect to the sale by us of 29,882,000 shares of our common stock in this offering based on the offering price of $5.02 per share, and after deducting underwriting discounts and commissions and estimated offering expenses payable by us, our as adjusted net tangible book value as of June 30, 2022 would have been $363.3 million, or $4.29 per share of common stock. This represents an immediate increase in net tangible book value of $0.23 per share to our existing stockholders and an immediate dilution of $0.73 in net tangible book value per share to new investors purchasing common stock in this offering. Dilution per share to new investors is determined by subtracting as adjusted net tangible book value per share after this offering from the public offering price per share paid by new investors.

The following table illustrates this calculation on a per share basis.

The foregoing table and calculations (other than the historical net tangible book value calculations) are based on 54,844,080 shares of our common stock outstanding as of June 30, 2022 and exclude as of such date:

To the extent that outstanding options or restricted stock awards outstanding as of June 30, 2022 have been or may be exercised or other shares issued, investors purchasing our common stock in this offering may

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experience further dilution. In addition, we may choose to raise additional capital due to market conditions or strategic considerations even if we believe we have sufficient funds for our current or future operating plans. To the extent that additional capital is raised through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our stockholders.

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Certain United States federal income tax consequences to non-U.S. holders

The following is a summary of certain United States federal income tax consequences of the ownership and disposition of our common stock. This summary deals only with common stock that is held as a capital asset by a non-U.S. holder (as defined below).

A “non-U.S. holder” means a beneficial owner of our common stock (other than an entity or arrangement treated as a partnership for United States federal income tax purposes) that is not, for United States federal income tax purposes, any of the following:

This summary is based upon provisions of the Internal Revenue Code of 1986, as amended (the “Code”), and regulations, rulings and judicial decisions as of the date hereof. Those authorities may be changed, perhaps retroactively, so as to result in United States federal income tax consequences different from those summarized below.

This summary does not address all of the United States federal income tax consequences that may be relevant to you in light of your particular circumstances, nor does it address the Medicare tax on net investment income, United States federal estate and gift taxes or the effects of any state, local or non-United States tax laws. In addition, it does not represent a detailed description of the United States federal income tax consequences applicable to you if you are subject to special treatment under the United States federal income tax laws (including if you are a United States expatriate, foreign pension fund, “controlled foreign corporation,” “passive foreign investment company” or a partnership or other pass-through entity for United States federal income tax purposes). We cannot assure you that a change in law will not alter significantly the tax considerations that we describe in this summary.

If a partnership (or other entity or arrangement treated as a partnership for United States federal income tax purposes) holds our common stock, the tax treatment of a partner generally will depend upon the status of the partner and the activities of the partnership. If you are a partnership or a partner of a partnership considering an investment in our common stock, you should consult your tax advisors.

If you are considering the purchase of our common stock, you should consult your own tax advisors concerning the particular United States federal income tax consequences to you of the ownership and disposition of our common stock, as well as the consequences to you arising under other United States federal tax laws and the laws of any other taxing jurisdiction.

Dividends

In the event that we make a distribution of cash or other property (other than certain pro rata distributions of our stock) in respect of our common stock, the distribution generally will be treated as a dividend for United States federal income tax purposes to the extent it is paid from our current or accumulated earnings and profits, as determined under United States federal income tax principles. Any portion of a distribution that exceeds our

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current and accumulated earnings and profits generally will be treated first as a tax-free return of capital, causing a reduction in the adjusted tax basis of a non-U.S. holder’s common stock, and to the extent the amount of the distribution exceeds a non-U.S. holder’s adjusted tax basis in our common stock, the excess will be treated as gain from the disposition of our common stock (the tax treatment of which is discussed below under “—Gain on Disposition of Common Stock”).

Dividends paid to a non-U.S. holder generally will be subject to withholding of United States federal income tax at a 30% rate or such lower rate as may be specified by an applicable income tax treaty. However, dividends that are effectively connected with the conduct of a trade or business by the non-U.S. holder within the United States (and, if required by an applicable income tax treaty, are attributable to a United States permanent establishment) are not subject to the withholding tax, provided certain certification and disclosure requirements are satisfied. Instead, such dividends are subject to United States federal income tax on a net income basis generally in the same manner as if the non-U.S. holder were a United States person as defined under the Code. Any such effectively connected dividends received by a foreign corporation may be subject to an additional “branch profits tax” at a 30% rate or such lower rate as may be specified by an applicable income tax treaty.

A non-U.S. holder who wishes to claim the benefit of an applicable treaty rate and avoid backup withholding, as discussed below, for dividends will be required (a) to provide the applicable withholding agent with a properly executed Internal Revenue Service (“IRS”) Form W-8BEN or Form W-8BEN-E (or other applicable form) certifying under penalty of perjury that such holder is not a United States person as defined under the Code and is eligible for treaty benefits or (b) if our common stock is held through certain foreign intermediaries, to satisfy the relevant certification requirements of applicable United States Treasury regulations. Special certification and other requirements apply to certain non-U.S. holders that are pass-through entities rather than corporations or individuals.

A non-U.S. holder eligible for a reduced rate of United States federal withholding tax pursuant to an income tax treaty may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS.

Gain on Disposition of Common Stock

Subject to the discussion of backup withholding below, any gain realized by a non-U.S. holder on the sale or other disposition of our common stock generally will not be subject to United States federal income tax unless:

A non-U.S. holder described in the first bullet point immediately above will be subject to tax on the gain derived from the sale or other disposition in the same manner as if the non-U.S. holder were a United States person as defined under the Code. In addition, if any non-U.S. holder described in the first bullet point immediately above is a foreign corporation, the gain realized by such non-U.S. holder may be subject to an additional “branch profits tax” at a 30% rate or such lower rate as may be specified by an applicable income tax treaty. An individual non-U.S. holder described in the second bullet point immediately above will be subject to a 30% (or such lower rate as may be specified by an applicable income tax treaty) tax on the gain derived from the sale or other disposition, which gain may be offset by United States source capital losses even though the individual is not considered a resident of the United States.

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Generally, a corporation is a “United States real property holding corporation” if the fair market value of its United States real property interests equals or exceeds 50% of the sum of the fair market value of its worldwide real property interests and its other assets used or held for use in a trade or business (all as determined for United States federal income tax purposes). We believe we are not and do not anticipate becoming a “United States real property holding corporation” for United States federal income tax purposes.

Information Reporting and Backup Withholding

Distributions paid to a non-U.S. holder and the amount of any tax withheld with respect to such distributions are required to be filed with the IRS. Copies of the information returns reporting such distributions and any withholding may also be made available to the tax authorities in the country in which the non-U.S. holder resides under the provisions of an applicable income tax treaty.

A non-U.S. holder will not be subject to backup withholding on distributions received if such holder certifies under penalty of perjury that it is a non-U.S. holder (and the payor does not have actual knowledge or reason to know that such holder is a United States person as defined under the Code), or such holder otherwise establishes an exemption.

Information reporting and, depending on the circumstances, backup withholding will apply to the proceeds of a sale or other disposition of our common stock within the United States or conducted through certain United States-related financial intermediaries, unless the beneficial owner certifies under penalty of perjury that it is a non-U.S. holder (and the payor does not have actual knowledge or reason to know that the beneficial owner is a United States person as defined under the Code), or such owner otherwise establishes an exemption.

Backup withholding is not an additional tax and any amounts withheld under the backup withholding rules will be allowed as a refund or a credit against a non-U.S. holder’s United States federal income tax liability provided the required information is timely furnished to the IRS.

Additional Withholding Requirements

Under Sections 1471 through 1474 of the Code (such Sections commonly referred to as “FATCA”), a 30% United States federal withholding tax may apply to any dividends paid on our common stock to (i) a “foreign financial institution” (as specifically defined in the Code and whether such foreign financial institution is the beneficial owner or an intermediary) that does not provide sufficient documentation, typically on IRS Form W-8BEN-E, evidencing either (x) an exemption from FATCA, or (y) its compliance (or deemed compliance) with FATCA (which may alternatively be in the form of compliance with an intergovernmental agreement with the United States) in a manner which avoids withholding, or (ii) a “non-financial foreign entity” (as specifically defined in the Code and whether such non-financial foreign entity is the beneficial owner or an intermediary) that does not provide sufficient documentation, typically on IRS Form W-8BEN-E, evidencing either (x) an exemption from FATCA, or (y) adequate information regarding certain substantial United States beneficial owners of such entity (if any). If a dividend payment is both subject to withholding under FATCA and subject to the withholding tax discussed above under “—Dividends,” an applicable withholding agent may credit the withholding under FATCA against, and therefore reduce, such other withholding tax. While withholding under FATCA would also have applied to payments of gross proceeds from the sale or other taxable disposition of our common stock, proposed United States Treasury regulations (upon which taxpayers may rely until final regulations are issued) eliminate FATCA withholding on payments of gross proceeds entirely. You should consult your own tax advisors regarding these requirements and whether they may be relevant to your ownership and disposition of our common stock.

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UNDERWRITING

Subject to the terms and conditions set forth in the underwriting agreement, between us and Jefferies LLC and Cowen and Company, LLC, as the representatives of the underwriters named below, we have agreed to sell to the underwriters, and each of the underwriters has agreed, severally and not jointly, to purchase from us, the respective number of shares of common stock shown opposite its name below:

The underwriting agreement provides that the obligations of the several underwriters are subject to certain conditions precedent such as the receipt by the underwriters of officers’ certificates and legal opinions and approval of certain legal matters by their counsel. The underwriting agreement provides that the underwriters will purchase all of the shares of common stock if any of them are purchased. If an underwriter defaults, the underwriting agreement provides that the purchase commitments of the nondefaulting underwriters may be increased or the underwriting agreement may be terminated. We have agreed to indemnify the underwriters and certain of their controlling persons against certain liabilities, including liabilities under the Securities Act, and to contribute to payments that the underwriters may be required to make in respect of those liabilities.

The underwriters have advised us that, following the completion of this offering, they currently intend to make a market in the common stock as permitted by applicable laws and regulations. However, the underwriters are not obligated to do so, and the underwriters may discontinue any market-making activities at any time without notice in their sole discretion. Accordingly, no assurance can be given as to the liquidity of the trading market for the common stock, that you will be able to sell any of the common stock held by you at a particular time or that the prices that you receive when you sell will be favorable.

The underwriters are offering the shares of common stock subject to their acceptance of the shares of common stock from us and subject to prior sale. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Commission and Expenses

The underwriters have advised us that they propose to offer the shares of common stock to the public at the public offering price set forth on the cover page of this prospectus and to certain dealers, which may include the underwriters, at that price less a concession not in excess of $0.18072 per share of common stock. After the offering, the public offering price and concession to dealers may be reduced by the representatives. No such reduction will change the amount of proceeds to be received by us as set forth on the cover page of this prospectus supplement.

The following table shows the public offering price, the underwriting discounts and commissions that we are to pay the underwriters and the proceeds, before expenses, to us in connection with this offering.

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We estimate expenses payable by us in connection with this offering, other than the underwriting discounts and commissions referred to above, will be approximately $535,000. We have agreed to reimburse the underwriters for certain expenses in an amount up to $10,000.

Listing

Our common stock is listed on the Nasdaq Global Select Market under the symbol “ALLK.”

No Sales of Similar Securities

We have agreed that, for a period of 90 days from the date of this prospectus supplement, we will not, without the prior written consent of Jefferies LLC and Cowen and Company, LLC, offer, sell, contract to sell, pledge, grant any option to purchase, make any short sale or otherwise transfer or dispose of, directly or indirectly, or file with the SEC a registration statement relating to, shares of our common stock, including but not limited to any options or warrants to purchase shares of our common stock or any securities that are convertible into or exchangeable for, or that represent the right to receive, shares of our common stock (collectively, “lock-up securities”), or publicly disclose the intention to make any such offer, sale, pledge, disposition or filing. We also will not, without the prior consent of Jefferies LLC and Cowen and Company, LLC, enter into any swap or other agreement that transfers, in whole or in part, any of the economic consequences of ownership of lock-up securities. The foregoing restrictions do not apply to:

(A) the shares of our common stock to be sold by us in this offering;

(B) the issuance by us of shares of common stock upon the exercise or settlement of options pursuant to our equity plans that are described in this prospectus supplement, or upon the conversion of convertible securities outstanding as of the date of the underwriting agreement and as described in this prospectus supplement;

(C) the issuance by us of lock-up securities pursuant to our stock plans that are described in this prospectus supplement;

(D) the issuance by us of lock-up securities in connection with (1) the acquisition by us of the business, technology, not less than a majority or controlling portion of the securities, property or other assets of another person or entity or pursuant to an employee benefit plan assumed by us in connection with such acquisition and the issuance of any such securities pursuant to any such agreement, or (2) our bona fide commercial transactions (including joint ventures, commercial relationships or other strategic transactions); or

(E) the filing of any registration statement on Form S-8 relating to securities granted or to be granted pursuant to our stock plans that are described in this prospectus supplement or any assumed employee benefit plan contemplated by clause (D) above.

The aggregate number of shares of common stock that we may sell or issue or agree to sell or issue pursuant to clause (D) above and, with respect to securities to be granted pursuant to any assumed employee benefit plan, pursuant to clause (E) above, shall not exceed 7.5% of the total number of shares of our common stock outstanding immediately following this offering. In the case of clauses (C) through (E) above, each recipient of such securities shall execute and deliver to the representatives, on or prior to the issuance of such securities, a lock-up agreement substantially to the effect set forth above.

Additionally, our executive officers and directors and certain stockholders affiliated with our directors have entered into lock-up agreements pursuant to which, for a period of 60 days from the date of this prospectus supplement, we and they will not, without the prior written consent of Jefferies LLC and Cowen and Company, LLC, offer, sell, contract to sell, pledge, grant any option to purchase, make any short sale or otherwise dispose of any lock-up securities. The foregoing is subject to several exceptions including:

(A) The following transfers of lock-up securities:

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In the case of any transfer pursuant to (i), (ii), (iii), (v), (ix) and (xi) above, the donee, transfereer or distributee must agree in writing to be bound by the lock-up restrictions. In the case of any transfer pursuant to (i), (ii), (iii), (iv) and (v) above, no filing under Section 16 of the Exchange Act or other public filing, report or announcement reporting a reduction in beneficial ownership of shares of common stock shall be required or voluntarily made during the lock-up period (other than a required filing on Form 5, Schedule 13G (or Schedule 13G/A) or Schedule 13F). In the case of (vi) above, if the lock-up signatory is required to

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file a report under Section 16 of the Exchange Act during the lock-up period, the lock-up signatory shall include a statement to the effect that such report relates to the circumstances described in (vi) above. In the case of (i), (ii), (iii), (v) and (ix) above, any transfer of lock-up securities must not involve a disposition for value. In the case of (vii) above, if the lock-up signatory is required to file a report under Section 16 of the Exchange Act during the lock-up period, the lock-up signatory shall include a statement in such report to the effect that such transfer is to us in connection with the repurchase of shares of common stock, as the case may be. In the case of (xiii) above, the lender taking beneficial or legal ownership of shares of common stock pursuant to such loan during the lock-up period shall agree in writing to be bound by the restrictions set forth in the lock-up agreement, and if the lock-up signatory is required to file a report under Section 16 of the Exchange Act during the lock-up period as a result of such loan, the lock-up signatory shall include a statement in such report to the effect that the lender will agree to be bound by the restrictions set forth in the lock-up agreement should the lender take beneficial or legal ownership of such shares of common stock during the lock-up period.

(B) Receipt from us of shares of common stock in connection with the exercise of options or other rights granted under a stock incentive plan or other equity award plan; or

(C) Entry into a written plan meeting the requirements of Rule 10b5-1 under the Exchange Act after the date of the lock-up agreement relating to the sale of the lock-up signatory’s shares, provided that (i) the securities subject to such plan may not be transferred until after the lock-up period expires and (ii) no public announcement or filing under the Exchange Act shall be voluntarily made regarding the establishment of such plan during the lock-up period.

Jefferies LLC and Cowen and Company, LLC may, in their sole discretion and at any time or from time to time before the termination of the 60-day period release all or any portion of the securities subject to lock-up agreements. There are no existing agreements between the underwriters and any of our shareholders who will execute a lock-up agreement providing consent to the sale of shares prior to the expiration of the lock-up period.

Stabilization

The underwriters have advised us that they may engage in short sale transactions, stabilizing transactions, or the imposition of penalty bids in connection with this offering. These activities may have the effect of stabilizing or maintaining the market price of the common stock at a level above that which might otherwise prevail in the open market.

The underwriters must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the shares of our common stock in the open market after pricing that could adversely affect investors who purchase in this offering.

A stabilizing bid is a bid for the purchase of shares of common stock on behalf of the underwriters for the purpose of fixing or maintaining the price of the common stock. A syndicate covering transaction is the bid for or

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the purchase of shares of common stock on behalf of the underwriters to reduce a short position incurred by the underwriters in connection with the offering. Similar to other purchase transactions, the underwriter’s purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market. A penalty bid is an arrangement permitting the underwriters to reclaim the selling concession otherwise accruing to a syndicate member in connection with the offering if the common stock originally sold by such syndicate member are purchased in a syndicate covering transaction and therefore have not been effectively placed by such syndicate member.

None of the underwriters make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. The underwriters are not obligated to engage in these activities and, if commenced, any of the activities may be discontinued at any time.

The underwriters may also engage in passive market making transactions in our common stock on the Nasdaq Global Select Market in accordance with Rule 103 of Regulation M during a period before the commencement of offers or sales of shares of our common stock in this offering and extending through the completion of distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker’s bid, that bid must then be lowered when specified purchase limits are exceeded.

Electronic Distribution

A prospectus in electronic format may be made available by e-mail or through online services maintained by one or more of the underwriters or their affiliates. In those cases, prospective investors may view offering terms online and may be allowed to place orders online. The underwriters may agree with us to allocate a specific number of shares of common stock for sale to online brokerage account holders. Any such allocation for online distributions will be made by the underwriters on the same basis as other allocations. Other than the prospectus in electronic format, the information on the underwriters’ web sites and any information contained in any other web site maintained by any of the underwriters is not part of this prospectus, has not been approved and/or endorsed by us or the underwriters and should not be relied upon by investors.

Other Activities and Relationships

The underwriter and certain of its affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. The underwriter and certain of its affiliates have, from time to time, performed, and may in the future perform, various commercial and investment banking and financial advisory services for us and our affiliates, for which they received or will receive customary fees and expenses.

Cowen and Company, LLC is the agent under our Sales Agreement, dated August 4, 2022, by and between us and Cowen and Company, LLC. Under the Sales Agreement, we may offer and sell, from time to time, shares of our common stock through Cowen and Company, LLC through an “at-the-market offering” as defined in Rule 415(a)(4) promulgated under the Securities Act. In addition, in the ordinary course of their various business activities, the underwriter and certain of its affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers, and such investment and securities activities may involve securities and/or instruments issued by us and our affiliates.

If the underwriters or their respective affiliates have a lending relationship with us, they routinely hedge their credit exposure to us consistent with their customary risk management policies. The underwriters and their respective affiliates may hedge such exposure by entering into transactions which consist of either the purchase

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of credit default swaps or the creation of short positions in our securities or the securities of our affiliates, including potentially the common stock offered hereby. Any such short positions could adversely affect future trading prices of the common stock offered hereby. The underwriters and certain of their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Disclaimers About Non-U.S. Jurisdictions

General

Other than in the United States, no action has been taken by us or the underwriters that would permit a public offering of the securities offered by this prospectus supplement in any jurisdiction where action for that purpose is required. The securities offered by this prospectus supplement may not be offered or sold, directly or indirectly, nor may this prospectus supplement or any other offering material or advertisements in connection with the offer and sale of any such securities be distributed or published in any jurisdiction, except under circumstances that will result in compliance with the applicable rules and regulations of that jurisdiction. Persons into whose possession this prospectus supplement comes are advised to inform themselves about and to observe any restrictions relating to the offering and the distribution of this prospectus supplement. This prospectus supplement does not constitute an offer to sell or a solicitation of an offer to buy any securities offered by this prospectus in any jurisdiction in which such an offer or a solicitation is unlawful.

Notice to Prospective Investors in the European Economic Area and the United Kingdom

In relation to each Member State of the European Economic Area and the United Kingdom, each a “Relevant State,” no shares have been offered or will be offered to the public in that Relevant State prior to the publication of a prospectus in relation to the shares which has been approved by the competent authority in that Relevant State or, where appropriate, approved in another Relevant State and notified to the competent authority in that Relevant State, all in accordance with the Prospectus Regulation, except that offers of shares may be made to the public in that Relevant State at any time under the following exemptions under the Prospectus Regulation:

provided that no such offer of shares shall require the issuer or any underwriter to publish a prospectus pursuant to Article 3 of the Prospectus Regulation or a prospectus pursuant to Article 23 of the Prospectus Regulation and each person who initially acquires any shares or to whom any offer is made will be deemed to have represented, acknowledged and agreed to and with each of the underwriters and us that it is a “qualified investor” within the meaning of Article 2(e) of the Prospectus Regulation. In the case of any shares being offered to a financial intermediary as that term is used in the Prospectus Regulation, each such financial intermediary will be deemed to have represented, acknowledged and agreed that the shares acquired by it in the offer have not been acquired on a non-discretionary basis on behalf of, nor have they been acquired with a view to their offer or resale to, persons in circumstances which may give rise to an offer of any shares to the public other than their offer or resale in a Relevant State to qualified investors as so defined or in circumstances in which the prior consent of the underwriters have been obtained to each such proposed offer or resale.

For the purposes of this provision, the expression an “offer to the public” in relation to any shares in any Member State means the communication in any form and by any means of sufficient information on the terms of the offer and any shares to be offered so as to enable an investor to decide to purchase or subscribe for any

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shares, and the expression “Prospectus Regulation” means Regulation (EU) 2017/1129. References to the Prospectus Regulation includes, in relation to the United Kingdom, the Prospectus Regulation as it forms part of United Kingdom domestic law by virtue of the European Union (Withdrawal) Act 2018.

Notice to Prospective Investors in the United Kingdom

In addition, in the United Kingdom, this document is being distributed only to, and is directed only at, and any offer subsequently made may only be directed at persons who are “qualified investors” (as defined in the Prospectus Regulation) (i) who have professional experience in matters relating to investments falling within Article 19 (5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended, or the Order and/or (ii) who are high net worth companies (or persons to whom it may otherwise be lawfully communicated) falling within Article 49(2)(a) to (d) of the Order, all such persons together being referred to as “relevant persons” or otherwise in circumstances which have not resulted and will not result in an offer to the public of the shares in the United Kingdom within the meaning of the Financial Services and Markets Act 2000.

Any person in the United Kingdom that is not a relevant person should not act or rely on the information included in this document or use it as basis for taking any action. In the United Kingdom, any investment or investment activity that this document relates to may be made or taken exclusively by relevant persons.

Notice to Prospective Investors in Canada

The shares may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the shares must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 of National Instrument 33-105 Underwriting Conflicts (NI 33-105), the underwriters are not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

Notice to Prospective Investors in Switzerland

The shares may not be publicly offered in Switzerland and will not be listed on the SIX Swiss Exchange, or SIX, or on any other stock exchange or regulated trading facility in Switzerland. This document does not constitute a prospectus within the meaning of, and has been prepared without regard to the disclosure standards for issuance prospectus under art. 652a or art. 1156 of the Swiss Code of Obligations or the disclosure standards for listing prospectus under art. 27 ff. of the SIX Listing Rules or the listing rules of any other stock exchange or regulated trading facility in Switzerland. Neither this document nor any other offering or marketing material relating to the shares or the offering may be publicly distributed or otherwise made publicly available in Switzerland.

Neither this document nor any other offering or marketing material relating to the offering, the Company, the shares have been or will be filed with or approved by any Swiss regulatory authority. In particular, this document will not be filed with, and the offer of shares will not be supervised by, the Swiss Financial Market

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Supervisory Authority, or FINMA, and the offer of shares has not been and will not be authorized under the Swiss Federal Act on Collective Investment Schemes, or CISA. The investor protection afforded to acquirers of interests in collective investment schemes under the CISA does not extend to acquirers of shares.

Notice to Prospective Investors in Australia

This prospectus:

The shares may not be directly or indirectly offered for subscription or purchased or sold, and no invitations to subscribe for or buy the shares may be issued, and no draft or definitive offering memorandum, advertisement or other offering material relating to any shares may be distributed in Australia, except where disclosure to investors is not required under Chapter 6D of the Corporations Act or is otherwise in compliance with all applicable Australian laws and regulations. By submitting an application for the shares, you represent and warrant to us that you are an Exempt Investor.

As any offer of shares under this document will be made without disclosure in Australia under Chapter 6D.2 of the Corporations Act, the offer of those securities for resale in Australia within 12 months may, under section 707 of the Corporations Act, require disclosure to investors under Chapter 6D.2 if none of the exemptions in section 708 applies to that resale. By applying for the shares you undertake to us that you will not, for a period of 12 months from the date of issue of the shares, offer, transfer, assign or otherwise alienate those shares to investors in Australia except in circumstances where disclosure to investors is not required under Chapter 6D.2 of the Corporations Act or where a compliant disclosure document is prepared and lodged with ASIC.

Notice to Prospective Investors in Dubai International Financial Centre, or DIFC

This document relates to an Exempt Offer in accordance with the Markets Rules 2012 of the Dubai Financial Services Authority, or the DFSA. This document is intended for distribution only to persons of a type specified in the Markets Rules 2012 of the DFSA. It must not be delivered to, or relied on by, any other person. The DFSA has no responsibility for reviewing or verifying any documents in connection with Exempt Offers. The DFSA has not approved this prospectus nor taken steps to verify the information set forth herein and has no responsibility for this document. The securities to which this document relates may be illiquid and/or subject to restrictions on their resale. Prospective purchasers of the securities offered should conduct their own due diligence on the securities. If you do not understand the contents of this document you should consult an authorized financial advisor.

In relation to its use in the DIFC, this document is strictly private and confidential and is being distributed to a limited number of investors and must not be provided to any person other than the original recipient, and may not be reproduced or used for any other purpose. The interests in the securities may not be offered or sold directly or indirectly to the public in the DIFC.

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Notice to Prospective Investors in Hong Kong

The shares have not been offered or sold and will not be offered or sold in Hong Kong, by means of any document, other than (a) to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571 of the Laws of Hong Kong), or the SFO, of Hong Kong and any rules made thereunder; or (b) in other circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32) of Hong Kong), or the CO, or which do not constitute an offer to the public within the meaning of the CO. No advertisement, invitation or document relating to the shares has been or may be issued or has been or may be in the possession of any person for the purposes of issue, whether in Hong Kong or elsewhere, which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) other than with respect to shares which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” as defined in the SFO and any rules made thereunder.

Notice to Prospective Investors in Japan

The shares have not been and will not be registered pursuant to Article 4, Paragraph 1 of the Financial Instruments and Exchange Act. Accordingly, none of the shares nor any interest therein may be offered or sold, directly or indirectly, in Japan or to, or for the benefit of, any “resident” of Japan (which term as used herein means any person resident in Japan, including any corporation or other entity organized under the laws of Japan), or to others for re-offering or resale, directly or indirectly, in Japan or to or for the benefit of a resident of Japan, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the Financial Instruments and Exchange Act and any other applicable laws, regulations and ministerial guidelines of Japan in effect at the relevant time.

Notice to Prospective Investors in Singapore

Each underwriter has acknowledged that this prospectus has not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, each underwriter has represented and agreed that it has not offered or sold any shares or caused the shares to be made the subject of an invitation for subscription or purchase and will not offer or sell any shares or cause the shares to be made the subject of an invitation for subscription or purchase, and has not circulated or distributed, nor will it circulate or distribute, this prospectus or any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the shares, whether directly or indirectly, to any person in Singapore other than:

Where the shares are subscribed or purchased under Section 275 of the SFA by a relevant person which is:

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securities or securities-based derivatives contracts (each term as defined in Section 2(1) of the SFA) of that corporation or the beneficiaries’ rights and interest (howsoever described) in that trust shall not be transferred within six months after that corporation or that trust has acquired the shares pursuant to an offer made under Section 275 of the SFA except:

Singapore SFA Product Classification—In connection with Section 309B of the SFA and the CMP Regulations 2018, unless otherwise specified before an offer of shares, we have determined, and hereby notify all relevant persons (as defined in Section 309A(1) of the SFA), that the shares are “prescribed capital markets products” (as defined in the CMP Regulations 2018) and Excluded Investment Products (as defined in MAS Notice SFA 04-N12: Notice on the Sale of Investment Products and MAS Notice FAA-N16: Notice on Recommendations on Investment Products).

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Legal matters

The validity of the issuance of our common stock offered in this prospectus supplement will be passed upon for us by Wilson Sonsini Goodrich & Rosati, Professional Corporation, Palo Alto, California. Davis Polk & Wardwell LLP, Menlo Park, California is acting as counsel for the underwriters.

Experts

Ernst & Young LLP, independent registered public accounting firm, has audited our financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2021, and effectiveness of our internal control over the financial reporting as of December 31, 2021, as set forth in their reports, which are incorporated by reference in this prospectus supplement and elsewhere in the registration statement. Our financial statements are incorporated by reference in reliance on Ernst & Young LLP’s reports, given on their authority as experts in accounting and auditing.

Where you can find more information

Because we are subject to the information and reporting requirements of the Exchange Act, we file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC’s website at www.sec.gov. We also maintain a website at investors.allakos.com where these materials are available. You may access these materials free of charge as soon as reasonably practicable after they are electronically filed with, or furnished to, the SEC. Except as specifically incorporated by reference herein, information contained on or accessible through our website is not a part of this prospectus supplement and is not incorporated by reference herein, and the inclusion of our website address in this prospectus supplement is an inactive textual reference only.

This prospectus supplement and the accompanying prospectus are part of a registration statement on Form S-3 that we filed with the SEC and do not contain all of the information in the registration statement. The full registration statement may be obtained from the SEC or us, as provided below. Forms of the indenture and other documents establishing the terms of the offered securities are or may be filed as exhibits to the registration statement. Statements in this prospectus supplement or the accompanying prospectus about these documents are summaries and each statement is qualified in all respects by reference to the document to which it refers. You should refer to the actual documents for a more complete description of the relevant matters. You may inspect a copy of the registration statement through the SEC’s website, as provided above.

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Incorporation by reference

The SEC allows us to incorporate by reference information into this prospectus supplement. This means that we can disclose important information to you by referring you to another document. Any information referred to in this way is considered part of this prospectus supplement from the date we file that document. Any reports filed by us with the SEC after the date of this prospectus supplement and before the date that the offering of the Common Stock by means of this prospectus supplement is terminated will automatically update and, where applicable, supersede any information contained in this prospectus supplement or incorporated by reference in this prospectus supplement.

We incorporate by reference in this prospectus supplement the documents set forth below that have been previously filed with the SEC; provided, however, that we are not incorporating any documents or information deemed to have been furnished rather than filed in accordance with SEC rules:

Notwithstanding the foregoing, we are not incorporating by reference any documents, portions of documents, exhibits or other information, whether specifically listed above or filed in the future, that is deemed to have been furnished to, rather than filed with, the SEC, including any information furnished pursuant to Items 2.02 or 7.01 of Form 8-K or related exhibits furnished pursuant to Item 9.01 of Form 8-K. You may request a free copy of any of the documents incorporated by reference in this prospectus (other than exhibits, unless they are specifically incorporated in the documents) by writing or telephoning us at the following address:

Allakos Inc.

825 Industrial Road, Suite 500.

San Carlos, CA 94070

(650) 597-5002

Exhibits to the filings will not be sent, however, unless those exhibits have specifically been incorporated by reference in this prospectus supplement and accompanying prospectus.

You should rely only on the information incorporated by reference or provided in this prospectus supplement. We have not authorized anyone else to provide you with different information.

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PROSPECTUS

Allakos Inc.

$250,000,000

Common Stock

We may offer and sell up to $250,000,000 in the aggregate of shares of our common stock from time to time in one or more offerings. In addition, selling stockholders, as may be named in one or more prospectus supplements, may offer and sell from time to time and in one or more offerings, shares of our common stock. We will determine when we sell shares of our common stock, which may be sold on a continuous or delayed basis directly to or through agents, dealers or underwriters as designated from time to time, or through a combination of these methods. We reserve the sole right to accept, and we and any agents, dealers and underwriters reserve the right to reject, in whole or in part, any proposed purchase of shares of our common stock. If any agents, dealers or underwriters are involved in the sale of any of shares of our common stock, the applicable prospectus supplement will set forth any applicable commissions or discounts payable to them. Our net proceeds from the sale of shares of our common stock also will be set forth in the applicable prospectus supplement. We also may provide investors with a free writing prospectus that includes this information. In addition, one or more selling stockholders to be named in a prospectus supplement hereto may offer and sell shares of our common stock from time to time, together or separately, in amounts, at prices and on terms that will be determined at the time of any such offering.

Each time that we or any selling stockholders sell shares of our common stock using this prospectus, we or any selling stockholders will provide a prospectus supplement and attach it to this prospectus if required. The prospectus supplement or a free writing prospectus will contain more specific information about the offering and the shares of our common stock being offered, including the prices and our net proceeds from the sales of such shares of our common stock. The selling stockholders will receive all of the proceeds from any sales of shares of our common stock owned by them. We will not receive any proceeds from the sale of shares of our common stock by selling stockholders. We may bear a portion of the expenses of the offering of shares of our common stock by one or more selling stockholders, except that the selling stockholders may pay any applicable underwriting fees, discounts or commissions and certain transfer taxes. The prospectus supplement or free writing prospectus may also add, update or change information contained in this prospectus. This prospectus may not be used to sell shares of our common stock unless accompanied by a prospectus supplement describing the method and terms of the offering.

You should carefully read this prospectus and any applicable prospectus supplement and free writing prospectus, together with any documents we incorporate by reference, before you invest in shares of our common stock.

INVESTING IN OUR COMMON STOCK INVOLVES RISKS. SEE “RISK FACTORS ” BEGINNING ON PAGE 2 OF THIS PROSPECTUS AND IN ANY SIMILAR SECTION CONTAINED IN OUR PROSPECTUS SUPPLEMENT CONCERNING FACTORS YOU SHOULD CONSIDER BEFORE INVESTING IN OUR COMMON STOCK.

Our common stock is listed on The Nasdaq Global Select Market (“NASDAQ”) under the symbol “ALLK.” On May 18, 2022, the closing price of our common stock, as reported on NASDAQ, was $2.96 per share.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

This prospectus is dated May 31, 2022

TABLE OF CONTENTS

Neither we nor any selling stockholder has authorized anyone to provide you with information other than that contained or incorporated by reference in this prospectus, in any accompanying prospectus supplement or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We and any selling stockholders take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. You should not assume that the information contained or incorporated by reference in this prospectus and any accompanying prospectus supplement or free writing prospectus is accurate as of any date other than the respective dates thereof. This prospectus and any accompanying prospectus supplement do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the registered securities to which they relate, nor do this prospectus and any accompanying prospectus supplement constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction.

i

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement on Form S-3 that we filed with the Securities and Exchange Commission (the “SEC”) using a “shelf” registration process. Under this shelf registration process, (i) we may, from time to time, offer and/or sell shares of our common stock in one or more offerings for an aggregate offering amount of up to $250,000,000 and/or (ii) one or more selling stockholders, if applicable, may, from time to time, offer and/or sell shares of our common stock in one or more resales. This prospectus provides you with a general description of the common stock that we and/or one or more selling stockholders may offer. Each time we sell shares of our common stock using this prospectus, we will provide a prospectus supplement and attach it to this prospectus and may also provide you with a free writing prospectus. The prospectus supplement and any free writing prospectus will contain more specific information about the offering, including the names of any selling stockholders, if applicable. The prospectus supplement may also add, update, change or clarify information contained in or incorporated by reference into this prospectus. Any statement that we make in this prospectus will be modified or superseded by any inconsistent statement made by us in a prospectus supplement. If there is any inconsistency between the information in this prospectus and the information in the prospectus supplement, you should rely on the information in the prospectus supplement.

The rules of the SEC allow us to incorporate by reference information into this prospectus. This means that important information is contained in other documents that are considered to be a part of this prospectus. Additionally, information that we file later with the SEC will automatically update and supersede this information. You should carefully read both this prospectus and the applicable prospectus supplement together with the additional information that is incorporated or deemed incorporated by reference in this prospectus as described under the heading “Information Incorporated by Reference” and any additional information described under the heading “Where You Can Find More Information” before making an investment in our common stock. This prospectus contains summaries of certain provisions contained in some of the documents described herein, but reference is made to the actual documents for complete information. All of the summaries are qualified in their entirety by the actual documents. Copies of the documents referred to herein have been filed, or will be filed or incorporated by reference, as exhibits to the registration statement of which this prospectus is a part. The registration statement, including the exhibits and documents incorporated or deemed incorporated by reference in this prospectus can be read on the SEC website mentioned under the headings “Information Incorporated by Reference” and “Where You Can Find More Information.”

THIS PROSPECTUS MAY NOT BE USED TO SELL ANY SECURITIES UNLESS ACCOMPANIED BY A PROSPECTUS SUPPLEMENT.

This prospectus incorporates by reference, and any prospectus supplement or free writing prospectus may contain and incorporate by reference, market data and industry statistics and forecasts that are based on independent industry publications and other publicly available information. Although we believe these sources are reliable, we do not guarantee the accuracy or completeness of this information and we have not independently verified this information. Although we are not aware of any misstatements regarding the market and industry data presented in this prospectus and the documents incorporated herein by reference, these estimates involve risks and uncertainties and are subject to change based on various factors, including those discussed under the heading “Risk Factors” contained or incorporated by reference in this prospectus, the applicable prospectus supplement and any related free writing prospectus and under similar headings in other documents that are incorporated by reference into this prospectus. Accordingly, investors should not place undue reliance on this information.

When we refer to “Allakos,” “we,” “our,” “us” and the “Company” in this prospectus, we mean Allakos Inc., unless the context indicates otherwise or unless otherwise specified. When we refer to “you,” we mean the holders of shares of our common stock.

ii

THE COMPANY

We are a clinical stage biotechnology company developing therapeutics which target immunomodulatory receptors present on immune effector cells involved in allergy, inflammatory and proliferative diseases. Activating these immunomodulatory receptors allows us to directly target cells involved in disease pathogenesis and, in the setting of allergy and inflammation, has the potential to result in broad inhibition of inflammatory cells. In the setting of proliferative diseases, blocking the inhibitory function of the receptors could restore the immune cell’s ability to identify and kill proliferative cells. Our most advanced antibodies are lirentelimab (AK002) and AK006. Lirentelimab selectively targets both mast cells and eosinophils, two types of white blood cells that are widely distributed in the body and play a central role in the inflammatory response. Inappropriately activated mast cells and eosinophils have been identified as key drivers in a number of severe diseases affecting the gastrointestinal tract, eyes, skin, lungs and other organs. We are developing lirentelimab for the treatment of eosinophilic esophagitis (“EoE”), eosinophilic gastritis (“EG”), eosinophilic duodenitis (“EoD”), atopic dermatitis, chronic spontaneous urticaria and potentially additional indications. Lirentelimab has received orphan disease status for EG, EoD, and EoE from the U.S. Food and Drug Administration (the “FDA”).

AK006 targets Siglec-6, an inhibitory receptor expressed selectively on mast cells. AK006 appears to have the potential to provide deeper mast cell inhibition than lirentelimab and, in addition to its inhibitory activity, reduce mast cell numbers. We plan to begin human studies with AK006 in the first half of 2023.

To date, lirentelimab has completed a Phase 2 study (ENIGMA 1) and Phase 3 study (ENIGMA 2) in patients with EG and/or EoD, a Phase 2/3 study in patients with EoE (KRYPTOS), as well as proof of concept studies in chronic spontaneous urticaria, severe allergic conjunctivitis, and indolent systemic mastocytosis.

The Phase 2 EG and/or EoD study with lirentelimab (ENIGMA 1) met all prespecified primary and secondary endpoints when compared to placebo and results were published in The New England Journal of Medicine. More recently, the ENIGMA 2 study met the histologic co-primary endpoint but failed to meet the symptomatic co-primary endpoint when compared to placebo. Similarly, the KRYPTOS study met the histologic co-primary endpoint but failed to meet the symptomatic co-primary endpoint when compared to placebo. After conducting post-hoc analyses, we believe that the trials missed their symptomatic co-primary endpoints due to the inclusion of mild patients and/or patients who had not failed standard of care. Although post-hoc analyses cannot be used to establish efficacy, these analyses can be helpful in generating hypothesis for future clinical studies. Based on these analyses, we believe that lirentelimab may have potential to treat the more severe EG/EoD and EoE patient populations. As a result, we plan to conduct additional studies with lirentelimab in these indications after discussions with the FDA.

We were incorporated in Delaware in March 2012. Our common stock is listed on the NASDAQ under the symbol “ALLK.” Our principal executive office is located at 825 Industrial Road, Suite 500, San Carlos, California 94070, and our telephone number is (650) 597-5002. Our website address is www.allakos.com. This website address is not intended to be an active link, and information on, or accessible through, our website should not be construed to be a part of this prospectus. We do, however, use our website as a means of disclosing material non-public information and for complying with our disclosure obligations under Regulation FD.

Additional information about us is included in documents incorporated by reference in this prospectus. See “Where You Can Find More Information” and “Incorporation by Reference.”

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RISK FACTORS

Investing in our common stock involves a high degree of risk. Before making a decision to invest in our common stock, in addition to carefully considering the other information contained in this prospectus, in any accompanying prospectus supplement and incorporated by reference herein or therein, you should carefully consider the risks described under the caption “Risk Factors” contained in the applicable prospectus supplement, and any related free writing prospectus, and the risks discussed under the caption “Risk Factors” contained in our most recent Annual Report on Form 10-K and in our most recent Quarterly Report on Form 10-Q, which are incorporated by reference in their entirety, together with other information in this prospectus, the prospectus supplement, the documents incorporated by reference herein and therein and any free writing prospectus that we may authorize for use in connection with a specific offering. See “Where You Can Find More Information” and “Incorporation by Reference.”

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus may contain, the documents incorporated by reference herein may contain and management may make certain “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical facts, may be forward-looking statements. Words such as “anticipate,” “believe,” “could,” “estimate,” “expects,” “intend,” “may,” “plan,” “potential,” “predict,” “project,” “should,” “will,” “would” or the negative of those terms, and similar expressions that convey uncertainty of future events or outcomes.

These statements include, but are not limited to, statements about:

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Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements that we make. These forward-looking statements involve risks and uncertainties that could cause our actual results to differ materially from those in the forward-looking statements, including, without limitation, the risks set forth under the heading “Risk Factors” in our most recent Annual Report on Form 10-K, our most recent Quarterly Report on Form 10-Q, any prospectus supplement hereto and in our other filings with the SEC. Moreover, we operate in a very competitive and rapidly changing environment. New risks emerge from time to time. It is not possible to predict all risks, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements we may make. Given these uncertainties, you should not place undue reliance on these forward-looking statements. We qualify all of the forward-looking statements in this prospectus and the documents incorporated by reference herein by these cautionary statements. Our forward-looking statements speak only as of the date of this prospectus or as of the date of the documents incorporated herein by reference. Except as required by law, we assume no obligation to update these forward-looking statements publicly, or to update the reasons actual results could differ materially from those anticipated in any forward-looking statements, whether as a result of new information, future events or otherwise.

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USE OF PROCEEDS

In the case of a sale of shares of our common stock by us, the use of proceeds will be specified in the applicable prospectus supplement. In the case of a sale of shares of our common stock by any selling stockholders, we will not receive any of the proceeds from such sale.

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SELLING STOCKHOLDERS

We may register the shares of our common stock for resale by certain selling stockholders. Information about selling stockholders, where applicable, including their identities, the number of shares of common stock registered and offered on their behalf, their beneficial ownership and their relationship with us will be set forth in a prospectus supplement, in a post-effective amendment or in documents incorporated by reference into this prospectus that we file with the SEC. No selling stockholder shall sell any shares of our common stock pursuant to this prospectus until we have identified such selling stockholder and the shares being offered for resale by such selling stockholder in a subsequent prospectus supplement or in a post-effective amendment. However, selling stockholders may sell or transfer all or a portion of their shares of our common stock pursuant to any available exemption from the registration requirements of the Securities Act.

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DESCRIPTION OF CAPITAL STOCK

The following descriptions of our capital stock and certain provisions of our amended and restated certificate of incorporation and amended and restated bylaws. The following description may not contain all of the information that is important to you. To understand the material terms of our common stock, you should read our amended and restated certificate of incorporation and amended and restated bylaws, copies of which have been filed with the SEC.

General

Our amended and restated certificate of incorporation authorizes capital stock consisting of:

As of March 31, 2022, we had 54,761,492 shares of our common stock issued and outstanding.

Common Stock

Voting Rights

Each holder of common stock is entitled to one vote for each share on all matters submitted to a vote of the stockholders, including the election of directors. Our certificate of incorporation and bylaws do not provide for cumulative voting rights. Because of this, the holders of a plurality of the shares of common stock entitled to vote in any election of directors can elect all of the directors standing for election, if they should so choose. With respect to matters other than the election of directors, at any meeting of the stockholders at which a quorum is present or represented, the affirmative vote of a majority of the voting power of the shares present in person or represented by proxy at such meeting and entitled to vote on the subject matter shall be the act of the stockholders, except as otherwise required by law. The holders of a majority of the stock issued and outstanding and entitled to vote, present in person or represented by proxy, shall constitute a quorum for the transaction of business at all meetings of the stockholders.

Dividends

Subject to preferences that may be applicable to any then-outstanding preferred stock, holders of our common stock are entitled to receive dividends, if any, as may be declared from time to time by our board of directors out of legally available funds.

Liquidation

In the event of our liquidation, dissolution or winding up, holders of our common stock will be entitled to share ratably in the net assets legally available for distribution to stockholders after the payment of all of our debts and other liabilities and the satisfaction of any liquidation preference granted to the holders of any then-outstanding shares of preferred stock.

Rights and Preferences

Holders of our common stock have no preemptive, conversion, subscription or other rights, and there are no redemption or sinking fund provisions applicable to our common stock. The rights, preferences and privileges of the holders of our common stock are subject to and may be adversely affected by the rights of the holders of shares of any series of preferred stock that we may designate in the future.

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Fully Paid and Nonassessable

All of our outstanding shares of common stock are fully paid and nonassessable.

Preferred Stock

Our board of directors has the authority, without further action by the stockholders, to issue up to 20,000,000 shares of preferred stock in one or more series and to fix the rights, preferences, privileges and restrictions thereof. These rights, preferences and privileges could include dividend rights, conversion rights, voting rights, redemption rights, liquidation preferences, sinking fund terms and the number of shares constituting any series or the designation of such series, any or all of which may be greater than the rights of our common stock. The issuance of preferred stock could adversely affect the voting power of holders of our common stock and the likelihood that such holders will receive dividend payments and payments upon liquidation. In addition, the issuance of preferred stock could have the effect of delaying, deferring or preventing change in our control or other corporate action. No shares of preferred stock are outstanding.

Registration Rights

Under our Amended and Restated Investors’ Rights Agreement, as amended, holders of a majority of our common stock or their transferees, have the right to require us to register the offer and sale of their shares, or to include their shares in any registration statement we file, in each case as described below.

Demand Registration Rights

The holders of a majority of our common stock are entitled to certain demand registration rights. At any time, the holders of at least 50% of the shares having registration rights then outstanding can request that we file a registration statement to register the offer and sale of their shares. We are only obligated to effect up to two such registrations. Each such request for registration must cover securities the anticipated aggregate public offering price of which, before deducting underwriting discounts and commissions, is at least $10 million. These demand registration rights are subject to specified conditions and limitations, including the right of the underwriters to limit the number of shares included in any such registration under certain circumstances. If we determine that it would be seriously detrimental to us and our stockholders to effect such a demand registration, we have the right to defer such registration, not more than twice in any twelve-month period, for a period of up to 120 days.

Form S-3 Registration Rights

The holders of a majority of our common stock are entitled to certain Form S-3 registration rights. At any time when we are eligible to file a registration statement on Form S-3, the holders of the shares having these rights then outstanding can request that we register the offer and sale of their shares of our common stock on a registration statement on Form S-3 so long as the request covers securities the anticipated aggregate public offering price of which is at least $1 million. These stockholders may make an unlimited number of requests for registration on a registration statement on Form S-3. However, we will not be required to effect a registration on Form S-3 if we have effected two such registrations within the twelve month period preceding the date of the request. Additionally, if we determine that it would be seriously detrimental to us and our stockholders to effect such a demand registration, we have the right to defer such registration, not more than twice in any twelve month period, for a period of up to 120 days.

Piggyback Registration Rights

The holders of a majority of our common stock are entitled to certain “piggyback” registration rights. If we propose to register the offer and sale of shares of our common stock under the Securities Act, all holders of these

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shares then outstanding can request that we include their shares in such registration, subject to certain marketing and other limitations, including the right of the underwriters to limit the number of shares included in any such registration statement under certain circumstances. As a result, whenever we propose to file a registration statement under the Securities Act, other than with respect to (1) a registration related to any employee benefit plan or a corporate reorganization or other transaction covered by Rule 145 promulgated under the Securities Act, (2) a registration in which the only stock being registered is common stock issuable upon conversion of debt securities also being registered, (3) a registration on any registration form that does not permit secondary sales or (4) a registration on any form that does not include substantially the same information as would be required to be included in a registration statement covering the sale of our common stock, the holders of these shares are entitled to notice of the registration and have the right, subject to certain limitations, to include their shares in the registration.

Expenses of Registration

We will pay all expenses relating to any demand registrations, Form S-3 registrations and piggyback registrations, subject to specified exceptions.

Termination

The registration rights terminate upon the earliest of (1) the date that is three years after the closing of our initial public offering and (2) as to a given holder of registration rights, the date when such holder of registration rights can sell all of such holder’s registrable securities during any ninety day period pursuant to Rule 144 promulgated under the Securities Act.

Anti-Takeover Effects of Certain Provisions of Delaware Law, Our Amended and Restated Certificate of Incorporation and Our Amended and Restated Bylaws

Certain provisions of Delaware law and certain provisions in our amended and restated certificate of incorporation and amended and restated bylaws summarized below may be deemed to have an anti-takeover effect and may delay, deter or prevent a tender offer or takeover attempt that a stockholder might consider to be in its best interests, including attempts that might result in a premium being paid over the market price for the shares held by stockholders.

Preferred Stock

Our amended and restated certificate of incorporation contains provisions that permit our board of directors to issue, without any further vote or action by the stockholders, shares of preferred stock in one or more series and, with respect to each such series, to fix the number of shares constituting the series and the designation of the series, the voting rights (if any) of the shares of the series and the powers, preferences or relative, participation, optional and other special rights, if any, and any qualifications, limitations or restrictions, of the shares of such series.

Classified Board

Our amended and restated certificate of incorporation provides that our board of directors is divided into three classes, designated Class I, Class II and Class III. Each class is an equal number of directors, as nearly as possible, consisting of one-third of the total number of directors constituting the entire board of directors. The term of the Class I directors will terminate on the date of the 2022 annual meeting, the term of the Class II directors shall terminate on the date of the 2023 annual meeting and the term of the Class III directors shall terminate on the date of the 2024 annual meeting. At each annual meeting of stockholders, successors to the class of directors whose term expires at that annual meeting will be elected for a three-year term.

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Removal of Directors

Our amended and restated certificate of incorporation provides that stockholders may only remove a director for cause by a vote of no less than a majority of the shares present in person or by proxy at the meeting and entitled to vote.

Director Vacancies

Our amended and restated certificate of incorporation authorizes only our board of directors to fill vacant directorships.

No Cumulative Voting

Our amended and restated certificate of incorporation provides that stockholders do not have the right to cumulate votes in the election of directors.

Special Meetings of Stockholders

Our amended and restated certificate of incorporation and amended and restated bylaws provide that, except as otherwise required by law, special meetings of the stockholders may be called only by an officer at the request of a majority of our board of directors, by the Chair of our board of directors or by our Chief Executive Officer.

Advance Notice Procedures for Director Nominations

Our bylaws provide that stockholders seeking to nominate candidates for election as directors at an annual or special meeting of stockholders must provide timely notice thereof in writing. To be timely, a stockholder’s notice generally will have to be delivered to and received at our principal executive offices before notice of the meeting is issued by the secretary of the company, with such notice being served not less than 90 nor more than 120 days before the meeting. Although the amended and restated bylaws do not give the board of directors the power to approve or disapprove stockholder nominations of candidates to be elected at an annual meeting, the amended and restated bylaws may have the effect of precluding the conduct of certain business at a meeting if the proper procedures are not followed or may discourage or deter a potential acquirer from conducting a solicitation of proxies to elect its own slate of directors or otherwise attempting to obtain control of the company.

Action by Written Consent

Our amended and restated certificate of incorporation and amended and restated bylaws provide that any action to be taken by the stockholders must be effected at a duly called annual or special meeting of stockholders and may not be effected by written consent.

Amending our Certificate of Incorporation and Bylaws

Our amended and restated certificate of incorporation may be amended or altered in any manner provided by the Delaware General Corporation Law (“DGCL”). Our amended and restated bylaws may be adopted, amended, altered or repealed by stockholders only upon approval of at least majority of the voting power of all the then outstanding shares of the common stock, except for any amendment of the above provisions, which would require the approval of a two-thirds majority of our then outstanding common stock. Our amended and restated certificate of incorporation provides that our bylaws may be amended, altered or repealed by the board of directors.

Authorized but Unissued Shares

Our authorized but unissued shares of common stock and preferred stock will be available for future issuances without stockholder approval, except as required by the listing standards of NASDAQ, and could be utilized for a

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variety of corporate purposes, including future offerings to raise additional capital, acquisitions and employee benefit plans. The existence of authorized but unissued and unreserved common stock and preferred stock could render more difficult or discourage an attempt to obtain control of the company by means of a proxy contest, tender offer, merger or otherwise.

Exclusive Jurisdiction

Our amended and restated certificate of incorporation provides that, unless we consent to the selection of an alternative forum, the Court of Chancery of the State of Delaware shall be the sole and exclusive forum for any derivative action or proceeding brought on our behalf, any action asserting a claim of breach of fiduciary duty, any action asserting a claim arising pursuant to the DGCL, any action regarding our amended and restated certificate of incorporation or our amended and restated bylaws, or any action asserting a claim against us that is governed by the internal affairs doctrine. These provisions shall not apply to suits brought to enforce a duty or liability created by the Exchange Act or any other claim for which the federal courts have exclusive jurisdiction and our stockholders cannot waive compliance with federal securities laws and the rules and regulations thereunder. Our amended and restated certificate of incorporation provides further that the federal district courts of the United States of America is the exclusive forum for resolving any complaint asserting a cause of action arising under the Securities Act. However, it is possible that a court could find our forum selection provisions to be inapplicable or unenforceable.

Business Combinations with Interested Stockholders

We are governed by Section 203 of the DGCL. Subject to certain exceptions, Section 203 of the DGCL prohibits a public Delaware corporation from engaging in a business combination (as defined in such section) with an “interested stockholder” (defined generally as any person who beneficially owns 15% or more of the outstanding voting stock of such corporation or any person affiliated with such person) for a period of three years following the time that such stockholder became an interested stockholder, unless (i) prior to such time the board of directors of such corporation approved either the business combination or the transaction that resulted in the stockholder becoming an interested stockholder; (ii) upon consummation of the transaction that resulted in the stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of such corporation at the time the transaction commenced (excluding for purposes of determining the voting stock of such corporation outstanding (but not the outstanding voting stock owned by the interested stockholder) those shares owned (A) by persons who are directors and also officers of such corporation and (B) by employee stock plans in which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer); or (iii) at or subsequent to such time the business combination is approved by the board of directors of such corporation and authorized at a meeting of stockholders (and not by written consent) by the affirmative vote of at least 66 2/3% of the outstanding voting stock of such corporation not owned by the interested stockholder.

Our amended and restated certificate of incorporation and our amended and restated bylaws provide that we must indemnify our directors and officers to the fullest extent authorized by the DGCL. We are expressly authorized to, and do, carry directors’ and officers’ insurance providing coverage for our directors, officers and certain employees for some liabilities. We believe that these indemnification provisions and insurance are useful to attract and retain qualified directors and executive directors.

The limitation on liability and indemnification provisions in our certificate of incorporation and bylaws may discourage stockholders from bringing a lawsuit against directors for breach of their fiduciary duty. These provisions may also have the effect of reducing the likelihood of derivative litigation against directors and officers, even though such an action, if successful, might otherwise benefit us and our stockholders. In addition, your investment may be adversely affected to the extent we pay the costs of settlement and damage awards against directors and officers pursuant to these indemnification provisions.

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Listing

Our common stock is listed on the NASDAQ under the symbol “ALLK.”

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is American Stock Transfer & Trust Company, LLC. The transfer agent and registrar’s address is 6201 15th Avenue, Brooklyn, New York 11219.

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PLAN OF DISTRIBUTION

We and/or one or more selling stockholders, if applicable, may sell the shares of common stock covered by this prospectus in any of the following ways (or in any combination):

We will prepare a prospectus supplement or supplements, if required, that will describe the method of distribution and disclose the terms and conditions of any offering of shares of common stock, including:

Any offering price and any discounts, commissions, concessions or agency fees allowed or reallowed or paid to dealers may be changed from time to time.

Pursuant to our Amended and Restated Investors’ Rights Agreement with certain stockholders, as amended, we will pay certain registration expenses of such selling stockholders.

We and/or any selling stockholders, if applicable, may distribute the common stock from time to time in one or more transactions at:

Underwriters, dealers or any other third parties described above may offer and sell the offered common stock from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. If underwriters or dealers are used in the sale of any shares of common stock, the common stock will be acquired by the underwriters or dealers for their own account and may be resold from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The common stock may be either offered to the public through underwriting syndicates represented by managing underwriters, or directly by underwriters. Generally, the underwriters’ obligations to purchase the shares of common stock will be subject to certain conditions precedent. The underwriters will be obligated to purchase all of the shares of common stock if they purchase any of the shares of common stock (other than any shares of common stock purchased upon exercise of any overallotment option), unless otherwise specified in the prospectus supplement. We and/or one or more selling stockholders may use underwriters with whom we and/or such selling stockholders have a material relationship. We will describe the nature of any such relationship in the prospectus supplement, as required, naming the underwriter.

We and/or one or more selling stockholders, if applicable, may sell the common stock through agents from time to time. The prospectus supplement will name any agent involved in the offer or sale of the common stock and

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any commissions paid to them. Generally, any agent will be acting on a best efforts basis for the period of its appointment. We may engage in “at the market” offerings into an existing trading market in accordance with Rule 415(a)(4) under the Securities Act. We may authorize underwriters, dealers or agents to solicit offers by certain purchasers to purchase the shares of common stock from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. These contracts will be subject only to those conditions set forth in the prospectus supplement, and the prospectus supplement will set forth any commissions to be paid for solicitation of these contracts. Any underwriters, dealers and agents that participate in the distribution of the common stock may be deemed to be “underwriters” as defined in the Securities Act. Any commissions paid or any discounts or concessions allowed to any such persons, and any profits they receive on resale of the common stock, may be deemed to be underwriting discounts and commissions under the Securities Act. We will identify any underwriters or agents and describe their compensation in a prospectus supplement.

The applicable prospectus supplement will set forth whether or not underwriters may over-allot or effect transactions that stabilize, maintain or otherwise affect the market price of the shares of common stock at levels above those that might otherwise prevail in the open market, including, for example, by entering stabilizing bids, effecting syndicate covering transactions or imposing penalty bids.

If at the time of any offering made under this prospectus a member of FINRA participating in the offering has a “conflict of interest” as defined in FINRA Rule 5121 (“Rule 5121”), that offering will be conducted in accordance with the relevant provisions of Rule 5121.

We and/or any selling stockholders may agree to indemnify an underwriter, dealer or agent against certain liabilities related to the selling of the common stock, including liabilities arising under the Securities Act.

The specific terms of the lock-up provisions in respect of any given offering will be described in the applicable prospectus supplement.

Selling stockholders may also sell securities under Rule 144 under the Securities Act, if available, or pursuant to other available exemptions from the registration requirements under the Securities Act, rather than under this prospectus. Registration of the shares of common stock covered by this prospectus does not mean that any shares of common stock will be offered or sold.

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LEGAL MATTERS

The validity of our common stock will be passed upon for us by Simpson Thacher & Bartlett LLP, Palo Alto, California. Additional legal matters may be passed upon for us or any underwriters, dealers or agents, by counsel that we will name in the applicable prospectus supplement.

EXPERTS

Ernst & Young LLP, independent registered public accounting firm, has audited our financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2021, and effectiveness of our internal control over the financial reporting as of December 31, 2021, as set forth in their reports, which are incorporated by reference in this prospectus and elsewhere in the registration statement. Our financial statements are incorporated by reference in reliance on Ernst & Young LLP’s reports, given on their authority as experts in accounting and auditing.

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WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. The SEC maintains an Internet web site that contains reports, proxy and information statements, and other information regarding issuers, including us, that file electronically with the SEC. The public can obtain any documents that we file electronically with the SEC at http://www.sec.gov.

Our website address is www.allakos.com. The information on our website, or that can be accessed through our website, however, is not, and should not be deemed to be, a part of this prospectus.

This prospectus is a part of the registration statement and does not contain all the information in the registration statement and the exhibits to the registration statement. Whenever a reference is made in this prospectus to a contract or other document of ours, the reference is only a summary and you should refer to the exhibits that are a part of the registration statement for a copy of the contract or other document. You may review a copy of the registration statement and the documents incorporated by reference herein through the SEC’s Internet website referred to above.

INCORPORATION BY REFERENCE

The SEC allows us to incorporate by reference information into this prospectus. This means that we can disclose important information to you by referring you to another document. Any information referred to in this way is considered part of this prospectus from the date we file that document. Any reports filed by us with the SEC after the date of this prospectus and before the date that the offering of the common stock by means of this prospectus is terminated will automatically update and, where applicable, supersede any information contained in this prospectus or incorporated by reference in this prospectus.

We incorporate by reference in this prospectus the documents set forth below that have been previously filed with the SEC; provided, however, that we are not incorporating any documents or information deemed to have been furnished rather than filed in accordance with SEC rules:

Notwithstanding the foregoing, we are not incorporating by reference any documents, portions of documents, exhibits or other information, whether specifically listed above or filed in the future, that is deemed to have been furnished to, rather than filed with, the SEC, including any information furnished pursuant to Items 2.02 or 7.01 of Form 8-K or related exhibits furnished pursuant to Item 9.01 of Form 8-K.

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You may request a free copy of any of the documents incorporated by reference in this prospectus (other than exhibits, unless they are specifically incorporated in the documents) by writing or telephoning us at the following address:

Allakos Inc.

825 Industrial Road, Suite 500

San Carlos, California 94070

(650) 597-5002

Exhibits to the filings will not be sent, however, unless those exhibits have specifically been incorporated by reference in this prospectus and any accompanying prospectus supplement.

You should rely only on the information incorporated by reference or provided in this prospectus. Neither we nor any selling stockholder has authorized anyone else to provide you with different information.

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29,882,000 Shares of Common Stock

PROSPECTUS SUPPLEMENT

September 19, 2022