The complement pathway is an important host defense system that contributes to both innate and acquired immunity. There are three pathways of complement activation: the classical pathway, lectin pathway and alternative pathway. Complement protein Factor I is a key serine protease that modulates the complement cascade by regulating the levels of C3 convertases. It circulates in plasma as a heavily N-glycosylated heterodimer made up of two disulfide linked chains, each carrying three N-linked oligosaccharide chains that may have both structural and functional roles in the interactions with the natural substrate and the cofactor during catalysis. Factor I is a serine protease with a high degree of specificity for C3b and C4b. It requires protein cofactors for cleavage of these complement proteins, Factor H, CR1 or MCP are required for C3b cleavage, and C4bp or CR1 are required for C4b cleavage.
Synonyms: Factor I light chain, AHUS3, C3b INA, C3b inactivator, C3B/C4B inactivator, C3BINA, CFAI_HUMAN, CFI, Complement component I, Complement control protein factor I, Complement factor I, Complement factor I light chain, F1, factor I, FactorI, FI, I factor, , KAF, Konglutinogen activating factor, Light chain of factor I, OTTHUMP00000219728.