Fig. 2.

Fluid phase and membrane-bound regulators of the complement system.

a FH is the most important fluid phase regulator of the complement system. FH acts as a cofactor for FI-mediated C3 cleavage. FH also prevents C3b from binding to the endothelial cell surface. In addition, FH facilitates the decay of the AP C3 convertase.

b Membrane bound regulators protect cells from (over)activation of complement on the cell surface. Complement regulator 1 (CD35) binds C3b and/or C4b and prevents the formation of C3 and C5 convertases, membrane co-factor protein (MCP, CD46) inactivates C3b and C4b by acting as a cofactor, decay-accelerating factor (DAF, CD55) accelerates the decay of the AP C3 convertase C3bBb, protectin (CD59) blocks the formation of MAC/C5b-9, and thrombomodulin (THBD, CD141) works as a cofactor for FH and C4b, resulting in the inactivation of C3b.

FI factor I, FH factor H, THBD/CD141 thrombomodulin