Exhibit 4.2

DELAWARE SEAL 2017 CORPORATE ALECTOR, INC. SECRETARY PRESIDENT AUTHORIZED SIGNATURE TRANSFER AGENT AND REGISTRAR (Brooklyn, NY) AMERICAN STOCK TRANSFER & TRUST COMPANY, LLC BY: COUNTERSIGNED AND REGISTERED: transferable on the books of the Corporation by the holder hereof in person or by duly authorized attorney upon surrender of this Certificate properly endorsed. This certificate is not valid unless countersigned and registered by the Transfer Agent and Registrar. WITNESS the facsimile seal of the Corporation and facsimile signatures of its duly authorized officers. Dated: ALECTOR, INC. ALECTOR, INC. FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK, PAR VALUE $0.0001 PER SHARE, OF is the registered holder of THIS CERTIFIES THAT CUSIP 014442 10 7 SEE REVERSE FOR CERTAIN DEFINITIONS INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE A

THE SIGNATURE(S) MUST BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION (BANKS, STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT TO S.E.C. RULE 17Ad-15. Signature(s) Guaranteed THE SIGNATURE(S) TO THIS ASSIGNMENT MUST CORRESPOND WITH THE NAME(S) AS WRITTEN UPON THE FACE OF THE CERTIFICATE IN EVERY PARTICULAR, WITHOUT ALTERATION OR ENLARGEMENT OR ANY CHANGE WHATEVER. NOTICE: Dated to transfer the said shares on the books of the within named Corporation with full power of substitution in the premises. Attorney Shares of the common stock represented by the within Certificate, and do hereby irrevocably constitute and appoint (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE) IDENTIFYING NUMBER OF ASSIGNEE PLEASE INSERT SOCIAL SECURITY OR OTHER For Value Received, hereby sell, assign and transfer unto Additional abbreviations may also be used though not in the above list. UNIF GIFT MIN ACT — Custodian (Cust) (Minor) under Uniform Gifts to Minors Act (State) as tenants in common as tenants by the entireties as joint tenants with right of survivorship and not as tenants in common ——— TEN COM TEN ENT JT TEN The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as though they were written out in full according to applicable laws or regulations: