The CD21 IHC test is a laboratory procedure that uses immunohistochemistry techniques to detect the presence and distribution of CD21, a surface marker expressed primarily on mature B lymphocytes and follicular dendritic cells (FDCs). CD21 plays a key role in the immune system by acting as a receptor for the complement component C3d and for the Epstein–Barr virus (EBV), facilitating B-cell activation and antibody responses.