Complement cascade with inhibitors. CR1, membrane cofactor protein (MCP/CD46), factor I (FI), factor H (FH), and CD55 are the major proteins that inhibit the APC. Of these, FH is the most critical. FH is a fluid-phase protein that can bind to cells via its interaction with cell surface glycosaminoglycan and sialic acid residues. It inhibits the APC in the fluid phase and on the cell surface. It competes with factor B for C3b attachment, thereby limiting formation of the C3 convertase, and it accelerates decay of the C3 convertase. Deficiency or impaired function of FH leaves healthy host cells and tissues highly vulnerable to complement-mediated attack. Complement FI functions as a complement inhibitor in the presence of its cofactors (FH, MCP/CD46, and CR1), to cleave and inactivate C3b (in the alternative pathway) and C4b (in the classical pathway and lectin pathway), blocking C3 convertase formation.¹  CD55 (also known as a decay-accelerating factor) is a cell surface glycosylphosphatidylinositol-anchored protein that inhibits the classical and alternative pathway C3 convertases and accelerates their decay. CD59 blocks terminal complement by preventing C9 from oligomerizing with the C5b-8 complex.