CR2 (CD21) is part of the B-cell co-receptor complex along with CD19 and CD81. It binds to antigens that have various breakdown products of C3, especially C3d, bound to them and, by cross-linking to the B-cell receptor, enhances sensitivity to antigen by at least a hundredfold. It is also used by the Epstein–Barr virus to invade B cells and produce the symptoms of infectious mononucleosis.