Complement Factor B

Complement factor B is 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 two fragments: Ba and Bb. Bb, a serine protease, then combines with C3b to form the AP C3 convertase (and, upon binding additional C3b, the C5 convertase). Bb has also been implicated in the proliferation and differentiation of preactivated B‑lymphocytes and the stimulation of lymphocyte blastogenesis. In contrast, Ba inhibits the proliferation of preactivated B‑lymphocytes and mediates the rapid spreading of peripheral blood monocytes. Through the convertases, factor B ultimately contributes to effector functions such as the lysis of erythrocytes.