Human complement component C3. Complement C3 contains two chains linked by a disulfide bond. Its activation by a C3 convertase releases the C3a anaphylatoxin from the amino end of the alpha chain and generates C3b, which associates with the Bb fragment of complement factor B to form the alternative-complement-pathway C3/C5 convertase.C3a anaphylatoxin is a vasoactive peptide and a mediator of inflammation. C3b, with its highly reactive thiol group, binds to the surface of foreign particles and facilitates phagocytosis. It binds to complement C5 and renders it susceptible to proteolysis by the classical-complement-pathway C3/C5 convertase. The activity of C3b is regulated by proteolytic cleavage involving factors H and I. Its degradation products can also be biologically active.