Abnormal B cell subset distribution Increased CD21low B cell proportion 2020-10-12T11:13:28Z Increased proportion autoreactive unresponsive CD21-/low B cells Complement receptor type 2 (CR2, CD21) is a 145-kD glycosylated single polypeptide chain consisting of a long extracellular domain of 15 to 16 short consensus repeat sequences, a transmembrane region, and a short cytoplasmic domain. The receptor is expressed on B cells, follicular dendritic cells, thymocytes and a subset of peripheral T cells. CD21 binds complement fragments C3d, C3dg and iC3b that are covalently bound to target antigens. On B cells, CD21 forms a complex together with CD19 and CD81, which functions as a coreceptor to the BCR. Upon simultaneous binding of complement-tagged antigens by CD21 and the BCR the threshold for B cell activation is reduced. Under these circumstances, CD21 acts as a bridge between the coreceptor complex and the BCR. Because CD21 augments B-cell receptor (BCR)-mediated signaling as part of the B-cell coreceptor complex, its downregulation may confer a state of anergy to these cells, as has been demonstrated among CD21-/low B cells in patients with rheumatoid arthritis, common variable immunodeficiency or hepatitis C associated cryoglobulinemia patients. These CD21-/low B cells are enriched in autoreactive clones that are unresponsive to BCR stimulation. Increased proportion relative to B-lymphocytes of a subset of B lymphocytes characterized by dim/low levels of CD21, i.e., CD21-/low, in flow cytometry, and additionally enriched in autoreactive clones as determined for instance by clonse showing rheumatoid factor (anti-IgG) reactivity and antibodies recognizing cytoplasmic and to a lesser extent nuclear structures.