CD21, also known as complement receptor 2, C3d receptor or Epstein-Barr virus (EBV) receptor, is a 140-kD membrane protein on B lymphocytes to which EBV binds during infection of these cells. CD21 is mainly expressed on follicular dendritic cells and mature B lymphocytes found in normal lymph node and tonsil tissue, and is absent on T lymphocytes, monocytes and granulocytes. Anti-CD21 is useful in identifying the follicular dendritic cell scaffold in various lymphomas, especially follicular lymphomas, and is also expressed in tumors of dendritic origin such as follicular dendritic cell sarcoma.