CD21 also known as complement C3d receptor, Epstein-Barr virus receptor, and complement receptor type 2 (CR2), is a protein that in humans is encoded by the CR2 gene. This protein is 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.It is expressed by follicular dendritic cells (FDC) and mature B cells, as well as by several types of epithelial cells. CD21 is useful in the identification of follicular dendritic cell matrix found in normal lymph node and tonsillar tissue, and also useful in identifying abnormal.