CD21 Antigen

The CD21 molecule is a 145 kDa type I transmembrane glycoprotein, which belongs to the family of complement regulatory proteins. CD21 is expressed by mature B lymphocytes and is rapidly lost following cellular activation. It is not expressed on early (pre-B) and late (B blast or plasma cell) B ontogeny. On follicular dendritic cells the CD21 antigen is strongly expressed. Expression of CD21 in a subset of T lymphocytes has been reported. Four types of CD21 ligands have been identified: They are products following proteolytic cleavage of the complement protein C3 (i. e. iC3b, C3dg and C3d), the Epstein-Barr (EBV) virus envelope, Interferon α (IFNα) and CD23. CD21 was found to be part of a large complex containing also CD19, and CD81 (also named TAPA-1) suggesting a key-role of CD21 during the immune response.