CD21 (Complement receptor type 2, CR2) is a receptor glycoprotein localized to the surface of B-cells. CD21 recognizes a number of ligands including CD23, interferon-alpha and complement component C3 fragments. Functionally, CD21 regulates B-cell activation and maturation and acts as a mediator between adaptive immunity and the complement system. In immunohistochemistry, CD21 shows positive membranous staining in mature B cells, plasma cells, follicular dendritic cells of the tonsil and lymph nodes, subsets of T-cells, and subsets of B cell lymphomas and angioimmunoblastic T-cell lymphoma. References: International Journal of Research in Medical Sciences. 2017;5:1156, DOI: 10.18203/2320-6012.ijrms20171218; Curr Protein Pept Sci. 2016;17(5):463-87, PMID: 26916158