Potent inhibitor of the complement membrane attack complex (MAC) action. Acts by binding to the C8 and/or C9 complements of the assembling MAC, thereby preventing incorporation of the multiple copies of C9 required for complete formation of the osmolytic pore (By similarity). Expressed in all tissues examined (liver, kidney, spleen, thymus, brain and heart). Low levels in thymus. Also expressed in mononuclear cells, erythrocytes and platelets. Barely detected in neutrophils.