| A | B |
|---|
| C1 (C1qr2s2) | Initiates the classical pathway |
| C1q | Binds to the Fc portion of antibody that has bound antigen, to apoptotic cells, and to cationic surfaces |
| C1r | Serine protease, cleaves C1s to make it an active protease |
| C1s | Serine protease, cleaves C4 and C2 |
| C4b | covalently binds to the surface of a microbe or cell, where antibody is bound and complement is activated |
| binds C2 for cleavage by C1s | C4b |
| stimulates inflammation -anaphylatoxin | C4a |
| C5b | initiates assembly of the membrane attack complex |
| C6 | Component of the MAC: binds to C5b and accepts C7 |
| C7 | Component of the MAC: binds to C5b,6 and inserts into lipid membranes |
| C8 | Component of the MAC: binds to C5b,6,7 and initiates the binding and polymerization of C9 |
| C9 | Component of the MAC: binds to C5b,6,7,8 and polymerizes to form membrane pores |
| C5a | stimulates inflammation (anaphylatoxin) |
| C1 inhibitor (C1 INH) | Serine protease inhibitor; binds to C1r and C1s and dissociates them from C1q |
| Factor I | Serine protease; cleaves C3b and C4b by using factor H, MCP, C4BP or CR1 as cofactors |
| Factor H | Binds C3b and displaces Bb |
| C4-binding protein (C4BP) | Binds C4b and displaces C2 |
| Membrane cofactor for protein (MCP CD46) | Leukocytes, epi-,endothelial cells Cofactor for factor I-mediated cleavage of C3b and C4b |
| Decay-accelerating factor (DAF) | Displaces C2b from C4b and Bb from C3b (dissociation of C3 convertases) |
| CD59 on Blood cells, endothelial cells, epithelial cells | binds C7, C8 and blocks C9 binding and prevents formation of the MAC |
| Properdin | Stabilizes C3 convertases (C3bBb) on microbial surfaces |
| Factor D | Plasma serine protease, cleaves factor B when it is bound to C3b |
| Factor B | Bb is a serine protease and the active enzyme of the C3 and C5 convertases |
| C3a | stimulates inflammation (anaphylatoxin) |
| binds to the surface of the microbe, where it functions as an opsonin and as a component of C3 and C5 convertases | C3b |
| Type 3 complement receptor (CR3, Mac-1, CD11b/CD18) function | Phagocytosis Leukocyte adhesion to endothelium (via ICAM-1) |
| Type 2 complement receptor (CR2, CD21) distribution | B lymphocytes, FDCs, nasopharyngeal epithelium |
| Type 2 complement receptor (CR2, CD21) ligands | C3d, C3dg > iC3b |
| Type 2 complement receptor (CR2, CD21) function | Coreceptor for B cell activation, Trapping of antigens in germinal centers, Receptor for EBV |
| Type 3 complement receptor (CR3, Mac-1, CD11b/CD18) | iC3b, ICAM-1; also binds microbes/ Mononuclear phagocytes, neutrophils, NK cells |
| Type 1 complement receptor (CR1, CD35) | C3b > C4b > iC3b, Mononuclear phagocytes, neutrophils, B and T cells, erythrocytes, eosinophils, FDCs |
| Type 1 complement receptor (CR1, CD35) | Phagocytosis, Clearance of immune complexes |
| Type 1 complement receptor (CR1, CD35) | Promotes dissociation of C3 convertases by acting as cofactor for cleavage of C3b, C4b |
