The complement factor B is part of the alternate pathway of the complement system. Factor B is cleaved by factor D into two fragments, Ba and BBovine, the latter being a serine protease that binds to complement component 3b (C3b) to generate the C3 or C5 convertase. Factor B is involved 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. Among the 15 variants of factor B gene, the variants His-9 and Gln-32 are associated with a reduced risk of age-related macular degeneration (ARMD). This antibody recognizes human complement factor B in indirect and sandwich ELISA.