Table 1.

Mammalian complement control proteins

Regulator	Distribution	Function
C1-inh	soluble	prevents spontaneous activation of C1 and inactivates C1 on surfaces
C4-bp*	soluble	accelerates decay of classical pathway C3/C5 convertase; factor I cofactor for degradation of C4b
Factor H*	soluble	accelerates decay of alternative pathway C3/C5 convertase; factor I cofactor for degradation of C3b
Factor I	soluble	cleaves C3b and C4b to inactive fragments when bound to a regulatory cofactor
CR1 (CD35)*	cell-bound	accelerates decay of classical and alternative C3/C5 convertase; factor I cofactor for degradation of C3b and C4b
MCP* (CD46)	cell-bound	cofactor for the cleavage of C3b and C4b by factor I
DAF* (CD55)	cell-bound	accelerates decay of C3/C5 convertases
CD59	cell-bound	prevents MAC formation

C1-inhibitor (C1-inh), C4 binding protein (C4-bp), factor H, and factor I are all soluble proteins that regulate the early stages of complement activation. Complement receptor 1 (CR1), membrane cofactor protein (MCP), and decay accelerating factor (DAF) are membrane-bound proteins that act at the level of C3, a central component of the complement cascade. CD59 is a cell surface protein that prevents formation of the terminal membrane attack complex (MAC) involved in membrane disruption and cell lysis. RCA proteins are indicated by an asterisk.