Table 2.
Complement regulators.
| Regulator | Interaction with complement |
|---|---|
| SUSD4 (Sushi domain–containing protein 4) | Classical and lectin pathway inhibited via binding to C1q |
| CSMD1 (CUB and Sushi multiple domains 1) | C3 convertase inhibited, MAC (C7 incorporation inhibited) |
| CD59 | (Cell membrane anchored) MAC (C7 incorporation inhibited) |
| CD55 (DAF, decay-accelerating factor) | (Cell membrane anchored but also in soluble form, eg, in synovial fluid) C3 convertase inhibited, accelerating the decay of C3/C5 convertases |
| CD46 (MCP, membrane cofactor protein) | (Cell membrane anchored) C3 convertase inhibited |
| CD35 (CR1, complement receptor 1) | (Cell membrane anchored) Acts on the C3b and C4b inactivation, immune complex processing and cleaning |
| C4b-binding protein (C4bp) | Inhibits the classical and the lectin pathways (C4) Can bind C3b, facilitates decay of C3 convertase Serves as cofactor for factor I which cleaves C4b and C3b |
| Properdin | (Soluble) Regulates alternative pathways by C3 convertase stabilization |
| Factor H (FH) | (Soluble) Inhibits alternative pathway |
| Factor I (FI) | Inactivation of C3b and C4b to iC3b and iC4b (CD35 acts as a cofactor) |
| C1 inhibitor (C1INH) | Can inactivate C1s, C1r (and kallikrein, plasmin, and coagulation factors XI, XII |
| Clusterin | Binds C7, C8, and C9b |
| Carboxypeptidase B | Inactivates C5a |
Abbreviations: C1r, complement C1r subcomponent; C1s, complement C1s subcomponent; C3, complement component C3; MAC, membrane attack complex.