The generation of the enzyme C3 convertase on the surface of a pathogen or cell is a crucial step in complement activation. The classical pathway C3 convertase is formed from membrane-bound C4b complexed with the protease C2b. The alternative pathway of complement activation uses a different but homologous C3 convertase, formed from membrane-bound C3b complexed with the protease Bb. These C3 convertases have the same activity, catalyzing the deposition of large numbers of C3b molecules that bind covalently to the pathogen surface, leading to opsonization and the activation of the effector cascade that causes membrane lesions.