sentences:  C4b has further functions.It interacts with protein C2; the same protease invoked earlier, C1s, then cleaves C2 into two parts, termed C2a and C2b, with C2b being released, and C2a remaining in association with C4b; the C4b-C2a complex of the two proteins then exhibits a further system-associated protease activity toward protein C3 (cleaving it), with subsequent release of both proteins, C4b and C2a, from their complex (whereupon C4b can bind another protein C2, and conduct these steps again).Because C4b is regenerated, and a cycle is created, the C4b-C2a complex with protease activity has been termed the "C3 convertase."