CD21, also known as CR2, is a 145 kDa transmembrane protein expressed primarily on B cells, where it is found in complex with other membrane proteins that promote normal humoral and cellular immune responses. CD21 expression is rapidly down-regulated during activation of B cells, and is also expressed on follicular dendritic cells and subsets of T cells. CD21 ligates 4 classes of ligands; complement component 3 proteolytic fragments iC3b, C3dg, and C3d; the Epstein-Barr virus glycoprotein gp350/220; the low-affinity IgE receptor (CD23), and the cytokine interferon alpha (IFNα). The primary role of CD21 is to function as a B cell co-receptor for antigen-mediated B cell activation through enhancement of signal transduction. This is achieved by co-ligation via C3d and surface IgM, where C3d is covalently linked to an antigen. IFNα has been shown to be a ligand of CD21, although the physiologic importance of this interaction is unknown.
Anti-Human CD21 (BL13)-152Sm—100 Tests