Table 4.
Key complement components and their functions.
| Classical pathway | |
|---|---|
| C1q | Binds to Fc region of Ab |
| C1r | Activates C1s |
| C1s | Activates C4/C2 |
| C2 | Key component of C4b2a |
| C4 | Key component of C4b2a |
| C4b2a | Actives C3 |
| C1 inhibitor | Inactivates C1r and C1s |
| C4BP | Blocks C4b2a formation |
| Factor I/CD46/CR1 | Inactivates C4b |
| CD55 | Prevents C4b2a formation |
| Lectin pathway | |
| MASP1 | Activates MASP2, MASP3, C4, and CoaC |
| MASP2 | Activates C4 |
| MASP3 | Activates alternative pathway |
| MBL | Binds to mannose residues on microbial surfaces |
| C2 | Key component of C4b2a |
| C4 | Key component of C4b2a |
| C4b2a | Actives C3 |
| Ficolin | Binds to carbohydrates of bacterial surfaces |
| Collectins | Binds to oligosaccharide structure or lipids of microorganic surfaces |
| C1 inhibitor | Inactivates MASP-1 and MASP-2 |
| C4BP | Blocks C4b2a formation |
| Factor I/CD46/CR1 | Inactivates C4b |
| CD55 | Prevents C4b2a formation |
| Alternative pathway | |
| C3(H2O) | Binds to factor B |
| Factor B | Key component of C3bBb and C3bBb3b |
| Factor D | Activates factor B |
| Properdin | Stabilizes C3bBb |
| C3bBb | Activates C3 |
| C4BP | Blocks C3bBb formation |
| Factor H | Inhibits C3bBb formation |
| Factor I/CD46/CR1/CRIg | Inactivates C3b |
| CD55 | Prevents C3bBb formation |
| Common pathway | |
| C3 | Key component of classical, lectin, and alternative pathways |
| Extrinsic pathway | |
| PMN-/MΦ-derived proteases | Activate C3 and C5 |
| TF | Activate C3 and C5 |
| CoaC factors (FIIa, IXa, Xa, XIIa, etc.) | Activate C3 and C5 |
| kallikrein | Activate C3 and C5 |
| FibC (plasmin, fibrin, etc.) | Activate C3 and C5 |
| Terminal pathway | |
| C5 | The first component (C5b) of C5b-9 |
| C6 | Binds to C5b and C7 |
| C7 | Binds to C8 |
| C8 | Bonds to C9 |
| C9 | Polymerized to form C5b-9 |
| C3bBb3b | Activates C5 |
| C4b2b3b | Activates C5 |
| CD59 | Inhibits C5b-9 formation |
| Vitronectin/clusterin | Inactivates C5b-9 |
C4BP, C4b binding protein; CoaC, coagulation cascade; CR1, complement receptor type 1; CRIg, complement receptor of immunoglobulin; Fib, fibrinolytic cascade; MΦ, macrophage; MASP, mannan-biding lectin serin protease; MBL, mannose binding lectin; PMN, polymorphonuclear leukocyte; TF, tissue factor.