Complement Factor B (CFB) belongs to the peptidase S1 family of enzymes. It is expressed by hepatocytes and macrophages and localizes to the major histocompatibility complex (MHC) class III region on chromosome 6. CFB which is a component of the alternate pathway of the complement system is cleaved by factor D into 2 fragments: Ba and Bb. Bb. The active subunit Bb is a serine protease which associates with C3b to form the alternative pathway C3 convertase. Bb is involved in the proliferation of preactivated B lymphocytes, while Ba inhibits their proliferation.