Complement factor B, a component of the alternative pathway (AP) of complement activation. Factor B is one of the upstream, proximal components unique to the AP and is essential for the activation of the pathway. It is an integral component that drives the amplification loop. It is cleaved by factor D into 2 fragments: Ba and Bb. Bb, a serine protease, then combines with complement factor 3b to generate the C3 or C5 convertase. It has also been implicated in proliferation and differentiation of preactivated B-lymphocytes, rapid spreading of peripheral blood monocytes, stimulation of lymphocyte blastogenesis and lysis of erythrocytes. Ba inhibits the proliferation of preactivated B-lymphocytes.