Complement receptor type 2 (CR2), also known as complement C3d receptor, Epstein-Barr virus receptor, and CD21 (cluster of differentiation 21), is a protein that in humans is encoded by the CR2 gene.
CR2 is involved in the complement system. It binds to iC3b (inactive derivative of C3b), C3dg, or C3d.[5]B cells express CR2 on their surfaces, allowing the complement system to play a role in B-cell activation and maturation. [6]