CD21,also known as Complement receptor type 2 (CR2),or complement C3d receptor,is expressed on all mature B-cells and follicular dendritic cells (FDCs),which 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.CD21 is the receptor for Epstein-Barr virus (EBV), and a useful marker for follicular dendritic cells in lymphoid tissue.