A patient with paroxysmal nocturnal hemoglobinuria (PNH) lacks GPI-anchored complement regulatory proteins. The absence of which two proteins specifically protects normal red cells from complement-mediated lysis?

• A CD46 (MCP) and CD35 (CR1)
• B CD55 (DAF) and CD59 (MIRL/protectin) ✓
• C Properdin and Factor H
• D C1-inhibitor and C4-binding protein

Explanation

PNH is caused by a somatic PIG-A mutation abolishing GPI anchor synthesis. The two key GPI-anchored complement regulatory proteins on red cells are CD55 (decay-accelerating factor, which degrades C3/C5 convertases) and CD59 (membrane inhibitor of reactive lysis, which blocks formation of the membrane attack complex at the C9 polymerisation step). Their absence leaves PNH red cells exquisitely sensitive to complement-mediated intravascular haemolysis. Factor H is a soluble regulator; C1-inhibitor and MCP/CR1 are not GPI-anchored.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.