Complement-Mediated Lysis of PNH Erythrocytes. The C3 convertase (left, green box) of the APC consists of a multiprotein complex of activated C3 and B (C3b and Bb), and the stabilizing cofactor P. The C5 convertase (right, green box) has the same components as the C3 convertase except that it includes 2 copies of C3b. The C3 and C5 convertases generate the active peptides C3a and C5a along with the MAC, which consists of C5b, C6, C7, C8, and multiple molecules of C9, and which functions as a cytolytic unit (blue box). The glycosyl-phosphatidylinositol (GPI)-anchored complement regulatory protein CD55/DAF restricts formation and stability of both the C3 and the C5 convertases (brown arrows), whereas GPI-anchored CD59/MIRL blocks formation of the MAC (brown arrow). Inhibition of MAC formation by anti-C5 monoclonal antibody eculizumab (gold arrow) ameliorates the intravascular hemolysis of PNH.