The steps in the alternative pathway can be summarized as follows:
Spontaneous hydrolysis of C3 to C3(H2O). Binding of factor B to C3(H2O), forming C3(H2O)B. Cleavage of factor B by factor D, forming C3(H2O)Bb, also known as fluid-phase C3 convertase. C3 convertase cleaves more C3 into C3a and C3b. C3b binds to pathogen surfaces and to factor B, forming C3bB, which is then cleaved by factor D to form C3bBb, the surface-bound C3 convertase. Amplification loop: C3bBb cleaves additional C3 molecules, leading to opsonization of pathogens and formation of the membrane attack complex (MAC).
