Complement Components and Factors
Nine serine proteases and their cofactors participate in the three pathways that lead to complement activation. C1r and C1s are the first two proteases that trigger the classical pathway. Three mannan-binding lectin-associated serine proteases (MASPs) are involved in the MBL pathway. C2 is involved in both the classical and the MBL pathways. Factor D is the first protease that activates the alternative pathway, which also includes proteases factor B and factor I. C3 plays a central role in both classical and alternative pathways. C3d, one of its fragments, binds to CR2. C5 is common to all three pathways, and C5a belongs to a family of related proteins known as anaphylatoxins.