Complement Receptor 2 (CD21)

CR2 and CD21, is a protein involved in the complement system. It binds to iC3b (inactive derivative of C3b), C3dg, or C3d.B cells have CR2 receptors on their surfaces, allowing the complement system to play a role in B-cell activation and maturation .CR2 on mature B cells form a complex with two other membrane proteins, CD19 and CD81(=TAPA-1). The CR2-CD19-CD81 complex is often called the B cell coreceptor complex,[3] because CR2 binds to antigens through attached C3d (or iC3b or C3dg) when the menbrane IgM binds to the antigen. This results in the B cell to have greatly enhanced response to the antigen.Epstein-Barr virus (EBV) binds to B cells at CR2 during infection of these cells. Yefenof et al. (1976) found complete overlapping of EBV receptors and C3 receptors on human B lymphocytes.