Alector Inc美股招股说明书(2026-05-07版)

$125,000,000 Common Stock We have entered into a sales agreement with TD Securities (USA) LLC (“TD Cowen”) relating to shares of our common stock offered by thisprospectus supplement and the accompanying prospectus. In accordance with the terms of the sales agreement, we may offer and sell shares of ourcommon stock having an aggregate offering price of up to $125,000,000 from time to time through or to TD Cowen acting as our agent or principal. Our common stock is listed on The Nasdaq Global Select Market under the symbol “ALEC.” On May 6, 2026, the last reported sale price of ourcommon stock was $2.57 per share. Sales of our common stock, if any, under this prospectus supplement and the accompanying prospectus will be made in negotiated transactions,including block trades or block sales, or by any method permitted by law deemed to be an “at the market offering” as defined in Rule 415 promulgatedunder the Securities Act of 1933, as amended (the “Securities Act”), including without limitation sales made through the Nasdaq Global Select Marketor on any other trading market for our common stock, or by any other method permitted by law. TD Cowen is not required to sell any specific amount ofsecurities, but will act as our sales agent using commercially reasonable efforts consistent with its normal trading and sales practices, on mutually agreedterms between TD Cowen and us. There is no arrangement for funds to be received in any escrow, trust, or similar arrangement. The compensation to TD Cowen for sales of common stock sold pursuant to the sales agreement will be an amount up to 3.0% of the gross proceeds ofany shares of common stock sold under the sales agreement. In connection with the sale of the common stock on our behalf, TD Cowen may be deemedto be an “underwriter” within the meaning of the Securities Act, and the compensation of TD Cowen may be deemed to be underwriting commissions ordiscounts. We have also agreed to provide indemnification and contribution to TD Cowen with respect to certain liabilities, including liabilities underthe Securities Act or the Securities Exchange Act of 1934, as amended (the “Exchange Act”). Our business and an investment in our common stock involve significant risks. These risks are described under thecaption “Risk Factors” beginning on page S-4 of this prospectus supplement and in the documents incorporated byreference into this prospectus supplement. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passedupon the accuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminaloffense. TD CowenMay 7, 2026 Table of Contents TABLE OF CONTENTS Prospectus SupplementAbout this Prospectus SupplementProspectus Supplement SummaryRisk FactorsSpecial Note Regarding Forward-Looking StatementsUse of ProceedsDilutionPlan of DistributionLegal MattersExpertsWhere You Can Find More InformationIncorporation of Certain Information by Reference ProspectusAbout this ProspectusThe CompanyRisk FactorsSpecial Note Regarding Forward-Looking StatementsUse of ProceedsDescription of SecuritiesDescription of Capital StockDescription of Debt Securities Description of Warrants Description of Purchase Contracts Plan of Distribution Legal Matters Experts Where You Can Find More Information Incorporation of Certain Information by Reference Table of Contents ABOUT THIS PROSPECTUS SUPPLEMENT This prospectus supplement and the accompanying prospectus are part of a registration statement that we have filed with the Securities andExchange Commission (the “SEC”), utilizing a “shelf” registration process. Under the shelf registration process, we may, from time to time, offer andsell any combination of our securities described in the accompanying prospectus in one or more offerings. We are providing information to you aboutthis offering of our common stock in two parts. The first part is this prospectus supplement, which provides you with specific information regarding theterms of this offering and our common stock, and also adds to and updates information contained in the accompanying prospectus and the documentsincorporated by reference in this prospectus supplement and the accompanying prospectus. The second part is the accompanying prospectus, whichprovides more general information, some of which does not apply to this offering of our common stock. We provide information to you about this offering of shares of our common stock in this prospectus supplement, which describes the specificdetails regarding this offering. This prospectus supplement adds to, updates, and changes information contained in the accompanying prospectus and theinformation incorporated by reference therein with respect to this offering of our common stock. To the extent that any information in this prospectussupplement is inconsistent with the information in the a