Critical role of complement in antibody mediated rejection in kidney transplantation

Table 1 List of regulatory proteins for complement system and their function

Soluble	Function	Pathway affected
C1 inhibitor	Dissociate C1r and C1s from C1q	Classical
C4 binding protein	Blocks formation of C3 convertase by binding C4b, Cofactor for cleavage of C4b by factor I	Classical, lectin
Factor H	Bind C3b, displacing Bb, Cofactor for factor I	Alternate
Factor I	Cleaves C4b and C3b aided by factor H, MCPC4BP or CR1	All 3 pathways
S protein	Binds soluble C567 and prevents its insertion into cell membrane	Terminal pathway
Anaphylatoxin inactivator	Inactivates anaphylatoxin activity of C3a and C5a by carboxypeptidase N-catalyzed removal of C-terminal Arg	Effector
Membrane bound
Membrane cofactor protein	Block formation of C3 convertase by binding C3b or C4b	All 3 pathways
Decay-accelerating factor	Accelerates dissociation of C4b2a and C3bBb	All 3 pathways
Complement receptor type 1	Bind C4b, displacing C2b or C3b displacing Bb; cofactor for I	All 3 pathways
Homologus restriction factor	Bind to C5b678 on autologous or allogenic cell blocking binding of C9	Terminal pathway