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. Expressed in all tissues tested (lung, testis liver, kidney, spleen, heart and skeletal muscle). Highest levels in lung and spleen, lowest levels in liver and skeletal muscle.