This gene encodes complement factor B a component of the alternative pathway of complement activation Factor B circulates in the blood as a single chain polypeptide Upon activation of the alternative pathway it is cleaved by complement factor D yielding the noncatalytic chain Ba and the catalytic subunit 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 This gene localizes to the major histocompatibility complex (MHC) class III region on chromosome 6 This cluster includes several genes involved in regulation of the immune reaction Polymorphisms in this gene are associated with a reduced risk of age-related macular degeneration The polyadenylation site of this gene is 421 bp from the 5 end of the gene for complement component 2