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RING finger protein 43
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This cell surface antigen is called also B2 (Weis et al, 1984). It is identical with the complement fragment C3d receptor known as CR2 [complement receptor-2] (C3dR [complement 3d receptor], C3d receptor). CD21 is the major receptor for C3d fragments on immune complexes. The terms CD21L and CD21S refer to long and short isoforms, respectively. Liu et al (1997) have reported that CD21L contains an additional exon (10a) and is expressed in follicular dendritic cells, whereas the short isoform lacking exon 10a is expressed in B-cells.
The human CR2 gene encodes a protein of 145 kDa and maps to chromosome 1q32 (Weis et al, 1987). CD21 is expressed on mature B-cells and follicular dendritic cells.
A soluble form of CD21 (sCD21), most likely containing only the extracellular portion of the molecule, is shed from the surface of peripheral B-cells and is found in human blood plasma. CD21 shedding is induced by stimulation with PMA plus calcium ionophore, or by stimulation of the B-cell receptor with anti-IgM+anti-CD40 (Masilamani et al, 2003).
CD21 is involved in the cell activation of B-cells and is engaged in homotypic aggregation of human B-lymphocytes (Bjšrck et al, 1993). On B-lymphocytes CD21 also binds IFN-alpha (Delcayre et al, 1991), and binds IFN-alpha in the same affinity range as CD23 and C3D (Asokan et al, 2006).
CD21 is also a ligand for another cell surface antigen, CD23, and regulates the synthesis of IgE (Aubry et al, 1992). CD21 also functions as a receptor for Epstein-Barr virus (EBV receptor, Epstein-Barr virus receptor) (Moore et al, 1987). The virus receptor found on human epithelial cells is identical with CD21 found on B-lymphocytes (Kief et al, 1992). CD21 and CD23 together with LFA-1 (CD11a / ICAM-1) appear to be the major molecules involved in homotypic aggregation of human B-cells.
For additional information on CD antigens see also: CD antigens MiniCOPE Dictionary.
See REFERENCES for entry CD21.
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