Factor B which is part of the alternate pathway of the complement system 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. Cleavage of Arg-|-Ser bond in complement component C3 alpha-chain to yield C3a and C3b, and Arg-|-Xaa bond in complement component C5 alpha-chain to yield C5a and C5b. Monomer (PubMed:19574954). Part of the C3-convertase enzyme complex comprised of Complement C3 beta chain (C3b) and Complement factor B Bb fragment (Bb) and CFP (PubMed:28264884, PubMed:31507604). Interacts to C3b; this interaction is dependent on the presence of Mg2+ (PubMed:28264884, PubMed:31507604). Interacts to CFP; this interaction contributes to the stabilization of the active C3-convertase enzyme complex (PubMed:31507604). The unliganded VWA domain has an inactive 'locked' conformation whereby the scissile Arg-259|Lys-260 bond is protected from proteolytic activation. Two major variants, F and S, and 2 minor variants, as well as at least 14 very rare variants, have been identified. Disease susceptibility may be associated with variants affecting the gene represented in this entry. Haplotype analyses have identified a statistically significant common risk haplotype and two protective haplotypes. CFB variant His-9 and C2 variant Asp-318, as well as CFB variant Gln-32 and a variant in intron 10 of C2, confer a significantly reduced risk of AMD. Disease susceptibility is associated with variants affecting the gene represented in this entry. Susceptibility to the development of atypical hemolytic uremic syndrome can be conferred by mutations in various components of or regulatory factors in the complement cascade system. Other genes may play a role in modifying the phenotype. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase S1 family.