Membrane immune adherence receptor that plays a critical role in the capture and clearance of complement-opsonized pathogens by erythrocytes and monocytes/macrophages (PubMed:2963069). Mediates the binding by these cells of particles and immune complexes that have activated complement to eliminate them from the circulation (PubMed:2963069). Also acts 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 (PubMed:2972794, PubMed:8175757). Also plays a role in immune regulation by contributing, upon ligand binding, to the generation of regulatory T cells from activated helper T cells (PubMed:25742728). {ECO:0000269|PubMed:25742728, ECO:0000269|PubMed:2963069, ECO:0000269|PubMed:2972794, ECO:0000269|PubMed:8175757}. (Microbial infection) Acts as a receptor for Epstein-Barr virus. . This is the function of ENSG00000203710 (CR1, complement C3b/C4b receptor 1 (Knops blood group)).