Factor I is a plasma protein of molecular weight 88 kDa, consisting of two disulphide linked chains of 50 and 38 kDa. Factor I is a serine protease with a high degree of specificity for C3b and C4b. It has a requirement for protein cofactors for cleavage of these complement proteins, Factor H, CR1 or MCP are required for C3b cleavage and C4bp or CR1 for C4b cleavage.
