The complement membrane attack complex (MAC, C5b-C9) causes cell lysis. Which complement regulatory protein on host cell surfaces specifically prevents MAC assembly by binding C8 and inhibiting C9 polymerization?

• A CD59 (protectin/MIRL) ✓
• B CD55 (decay-accelerating factor)
• C CD46 (membrane cofactor protein)
• D Factor H

Explanation

CD59 (protectin/membrane inhibitor of reactive lysis, MIRL) is a GPI-anchored protein that binds C8 and C9, preventing polymerization of C9 into the lytic MAC pore. Factor H and CD35 regulate C3/C5 convertase at earlier steps; CD55 (DAF) accelerates decay of C3/C5 convertases; CD46 acts as a cofactor for factor I-mediated cleavage of C3b. In paroxysmal nocturnal hemoglobinuria (PNH), loss of GPI-anchored proteins CD55 and CD59 causes complement-mediated hemolysis, underscoring the clinical importance of CD59 in protecting RBCs from MAC.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.