Table 1.

A: Soluble complement regulatory factors
REGULATOR	FUNCTION
C1 inhibitor	Serine protease that targets the C1s/C1r, inhibiting activation of C4 and C2 [19]
Factor H	Modulates C3b formation by acting as a co-factor for Factor I and by accelerating the decay of the C3 convertases [20]
Factor I (C3b /C4b inactivator)	Decreases complement activation by cleaving C3b and C4b when complexed with co-factors such as CD46 [21,22]
C4 Binding Protein (C4BP)	A co-factor for Factor I, binds C4b increasing proteolytic accessibility [23]
Vitronectin (S40)	Inhibits the terminal cascade and formation of the MAC; may have other roles in regulation of disease responses [24–26]
Clusterin (Apo J)	Similar to vitronectin, inhibits the formation of the MAC and may have other functions [26]

B: Membrane-bound complement regulatory factors
REGULATOR	FUNCTION
CD35 (Complement receptor 1, CR1)	Decay accelerating factor for C3/C5 convertases, facilitates phagocytosis of cells with complement activated, co-factor for Factor I, fixes complement immune complexes on erythrocytes, has limited tissue distribution in humans [27]
CD46 (Membrane cofactor protein, MCP)	Cofactor for factor I, regulator of T-cell differentiation and apoptosis, widely expressed in humans [16,28,29]
CD55 (Decay accelerating factor, DAF)	Inhibits formation and accelerates decay of C3 convertases [30–32]
CD59 (MAC inhibitory protein, MAC-IP, 20 kDa homologous restriction factor, HRF20)	Inhibits formation of the MAC by binding C5b/8 complex and interfering with insertion of C9 [33,34]