Table 1.
Non-exhaustive list of complement proteins and their activation products.
| Complement protein/s | Receptor | Function | Reference |
|---|---|---|---|
| C3a | C3aR | Anaphylatoxin (can cause smooth muscle contraction, vasodilation and increased histamine release by mast cells), CD4+ T cell activation, monocyte activation (via NLRP3 inflammasome) cancer progression (direct or indirect effect) | [63, 65, 147] |
| C4a | PAR1, PAR4 | Anaphylatoxin | [169] |
| C5a | C5aR1, C5aR2 | Anaphylatoxin, NLRP3 inflammasome activation in CD4+ T cells and monocytes, regulates cancer progression | [66] |
| C3b | CD46, FH, CR1 | Opsonin (via its receptors), part of the C3 and C5 convertases, activates CD4 T cells in autocrine fashion | [61] |
| Factor H (FH) | None, binds to Factor I and C3b or cell surface | In complex with the serine protease factor I, cleave and inactivate C3b to iC3b | [170] |
| CD46 | None, binds to C3b, C4b and Factor I | In complex with the serine protease factor I, cleave and inactivate C3b (or C4b) to iC3b, potent costimulatory molecule on CD4+ T cell activation and metabolism | [61, 62, 73] |
| Complement receptor 1 (CR1, CD35) | None, binds to C3b, iC3b | cleaves iC3b to C3dg, mediates inhibitory signals in T cell proliferation | [171] |
| Complement receptor 2 (CR2, CD21) | None, binds the degradation products of C3b (iC3b, C3dg, C3d) | Potent costimulatory molecule in B cells. Also controls survival (CR1) of germinal center B cells during development | [172–174] |
| Complement receptor 3 (CR3 or CD11b/CD18, or Mac-1) | None, binds iC3b | Opsonizes iC3b coated bacteria | [28, 175] |
| Complement receptor 4 (CR4 or CD11c/CD18) | None, binds to different part of iC3b than CR3 | Opsonizes iC3b coated bacteria | [28, 175] |