Factor B is a component of the alternative pathway of complement. It contains a serine protease (SP) domain, and when activated it provides the catalytic activity of the alternative pathway C3 and C5 convertases. Factor B is generated as a single-chain protein, and cleavage by factor D generates two peptide fragments (Ba and Bb). The Bb region (which contains the SP domain) remains bound to C3(H2O) and C3b, forming the alternative pathway convertases [C3(H2O)Bb and C3bBb].