TABLE 2.

Complement receptors and membrane-bound complement inhibitors

Category and proteinCharacteristic(s)
Complement ligands with complement-inhibitory function
    CR1Cofactor for factor I cleavage of C3b to iC3b and further to C3d and of C4b to C4d; binds to MBL and C1q; clearance of opsonized pathogens and C3b/C4b associated with immune complexes (“immune adherence”)
    CD46Cofactor for factor I cleavage of C3b and C4b; serves as receptor for pathogens such as measles virus and (?) N. gonorrhoeae
    CD55Accelerates decay of C3 convertase assembled on cells
    CD59Inhibits assembly of membrane attack complex (C9 polymerization)
Complement ligands
    CR2Binds primarily to C3d and C3dg; part of the CR2/CD19/CD81 complex that mediates B cell responses to antigens linked to C3 fragments
    CR3Ligand for iC3b; phagocytosis
    CR4Binds C3d/C3dg; function not known
    CRIgLigand for the β-chain of C3b/iC3b; role for pathogen clearance in mouse model
    C1qRLigand for C1q; phagocytosis
    SIGN-R1Complement receptor identified as one of the murine homologs of DC-SIGN; binds selected pneumococcal polysaccharides and C1q and can activate the classical pathway in an antibody-independent manner
Receptors for anaphylatoxins
    C3aRBinds C3a/C3a des-Arg; vasodilatation
    C5aRBinds C5a/C5a des-Arg; chemotaxis; possible modulatory role in airway inflammation