Membrane immune adherence receptor that plays a critical role in the capture and clearance of complement-opsonized pathogens by erythrocytes and monocytes/macrophages . Mediates the binding by these cells of particles and immune complexes that have activated complement to eliminate them from the circulation . Acts also in the inhibition of spontaneous complement activation by impairing the formation and function of the alternative and classical pathway C3/C5 convertases, and by serving as a cofactor for the cleavage by factor I of C3b to iC3b, C3c and C3d,g, and of C4b to C4c and C4d. Plays also a role in immune regulation by contributing, upon ligand binding, to the generation of regulatory T cells from activated helper T cells .