B cells are bursal or bone marrow-derived lymphocytes whose principle function in the adaptive immune system is to make antibodies against antigens. A simple definition of B lymphocytes is a population of cells that express clonally diverse cell surface immunoglobulin (Ig) receptors recognizing specific antigenic epitopes. Over 10 B cell-specific cell surface molecules have been identified by monoclonal antibodies, including CD19, which is considered a pan-B cell and possibly follicular dendritic cell marker, and CD20, which is considered to be a marker for mature B cells.

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