Exhibit 5.1

 

		Wilson Sonsini Goodrich & Rosati Professional Corporation   650 Page Mill Road Palo Alto, California 94304-1050   O: 650.493.9300 F: 866.974.7329

October 16, 2026

Alector, Inc.

131 Oyster Point Boulevard, Suite 600

South San Francisco, California 94080

Re: Alector, Inc. – Sale and Issuance of Shares of Common Stock

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-3, filed by Alector, Inc. (the “Company”) with the Securities and Exchange Commission (the “Commission”) on March 12, 2026 under the Securities Act of 1933, as amended (the “Securities Act”), and declared effective on April 30, 2026 (the “Registration Statement”), the prospectus contained within the Registration Statement (the “Base Prospectus”), and the prospectus supplement to the Registration Statement dated October 16, 2026 (together with the Base Prospectus, the “Prospectus”), in connection with the registration under the Securities Act of 36,600,000 shares of the Company’s common stock, $0.0001 par value per share (collectively, the “Shares”), to be issued and sold by the Company. The offering of the Shares is being made pursuant to that certain underwriting agreement, dated October 15, 2026, by and among the Company, Morgan Stanley & Co. LLC and BTIG, LLC (the “Underwriting Agreement”).

We are acting as counsel for the Company in connection with the issuance and sale of the Shares by the Company. In such capacity, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary for the purposes of rendering this opinion. In our examination, we have assumed (i) the authenticity of original documents and the genuineness of all signatures; (ii) the conformity to the originals of all documents submitted to us as copies; (iii) the truth, accuracy and completeness of the information, representations and warranties contained in the records, documents, instruments and certificates we have reviewed; (iv) that the Underwriting Agreement has been duly authorized and validly executed and delivered by the parties thereto (other than the Company); (v) that the Shares will be issued and sold in compliance with applicable U.S. federal and state securities laws and in the manner stated in the Registration Statement and the Prospectus; (vi) with respect to the Shares offered by the Company, that there will be sufficient shares of common stock authorized under the Company’s organizational documents that are not otherwise reserved for issuance; and (vii) the legal capacity of all natural persons.

Our opinion that any document is legal, valid and binding is qualified as to:

(a)  limitations imposed by bankruptcy, insolvency, reorganization, arrangement, fraudulent transfer, moratorium or other similar laws relating to or affecting the rights of creditors generally;

 

AUSTIN  BOSTON  BOULDER  BRUSSELS  CENTURY CITY  HONG KONG  LONDON  LOS ANGELES  NEW YORK  PALO ALTO

SALT LAKE CITY  SAN DIEGO  SAN FRANCISCO  SEATTLE  SHANGHAI  WASHINGTON, DC  WILMINGTON, DE

(b)  rights to indemnification and contribution, which may be limited by applicable law or equitable principles; and

(c)  the effect of general principles of equity, including without limitation concepts of materiality, reasonableness, good faith and fair dealing, and the possible unavailability of specific performance or injunctive relief, whether considered in a proceeding in equity or at law.

We express no opinion herein as to the laws of any state or jurisdiction other than the General Corporation Law of the State of Delaware (including the statutory provisions and all applicable judicial decisions interpreting those laws) and the federal laws of the United States of America.

On the basis of the foregoing, we are of the opinion that the Shares to be issued and sold by the Company have been duly authorized and, when such Shares are issued and paid for in accordance with the terms of the Underwriting Agreement, will be validly issued, fully paid and nonassessable.

We hereby consent to the filing of this opinion as an exhibit to the Company’s Current Report on Form 8-K filed on or about the date hereof for incorporation by reference into the Registration Statement, and to the use of our name wherever it appears in the Registration Statement, the Prospectus, and in any amendment or supplement thereto. In giving such consent, we do not believe that we are “experts” within the meaning of such term as used in the Securities Act or the rules and regulations of the Commission issued thereunder with respect to any part of the Registration Statement, including this opinion as an exhibit or otherwise.

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