CD21

CD21, also known as complement receptor 2 (CR2) or the C3d receptor, is a cell-surface glycoprotein that binds complement fragments. In humans, CD21 is expressed primarily on mature B lymphocytes and on follicular dendritic cells of secondary lymphoid tissues. It is a component of the B cell co-receptor complex, which also includes CD19 and CD81. The CD21–CD19–CD81 complex amplifies B cell receptor signaling and lowers the activation threshold when B cells encounter complement-tagged antigen.

CD21 binds complement fragments such as C3d, iC3b, and C3dg, enabling efficient capture and presentation of opsonized antigens by B cells. On follicular dendritic cells, CD21 helps retain immune complexes in germinal centers, supporting affinity maturation and humoral memory.

CD21 also serves as the entry receptor for Epstein-Barr virus, with the viral glycoproteins gp350/220 binding CD21 to attach and infect B cells, contributing to the B cell tropism of EBV.

In clinical and research settings, CD21 is used as a marker to identify B cell populations and follicular dendritic cells by flow cytometry or immunohistochemistry. Altered CD21 expression has been observed in certain immunological conditions and infections, and because of its role in EBV entry it is of interest in studies of viral pathogenesis and humoral immunity.

See also: CD19, CD81, Epstein-Barr virus, complement system.