Complement Activation via Alternative Pathway
For the alternative activation pathway, constant availability of C3b requires because this pathway does not rely on antigen-binding antibodies. Bacterial lipopolysaccharides may protect C3b due to the downregulation of natural C3b inhibitors (factor I and factor H). Free C3b binds factor B to produce C3bB, a substrate of a circulating enzyme, factor D, which cleaves B into Ba and Bb. Bb binds to C3b to give the enzyme C3bBb, the alternative pathway C3 convertase, stabilized by factor P (properdin), and then the new enzyme C3bBb3b, the alternative pathway C5 convertase, is constituted. The subsequent events are analogous to the classical pathway.