Complement Factor B

Complement factor B is the zymogen that powers the alternative pathway. Circulating as a single chain, it binds C3b in a Mg2+-dependent manner through its von Willebrand factor type A domain; factor D then cleaves it into the Ba fragment and the active Bb serine protease, and C3bBb constitutes the alternative-pathway C3 convertase. Its three CCP domains and C-terminal protease domain closely parallel the classical-pathway protease C2, reflecting their shared ancestry within the MHC class III region. Genetic variants of factor B influence susceptibility to age-related macular degeneration. Recombinant factor B enables reconstitution of alternative-pathway assays.