CD45R, also known as B220, a member of the protein tyrosine phosphate family and a major cell surface glycoprotein, represents a restricted form of the CD45 family which primarily recognizes only cells of B-lineage from pro-B cell through mature B lymphocytes and prior to the availability of anti-CD19 monoclonal antibodies was commonly used as a pan B-cell marker. It also reacts with certain activated T cells as well as non-MHC-restricted lytically active lymphokine-activated killer (LAK) cells. In vivo administration of RA3-6B2 has been shown to affect differentiation of both T and B cells in normal mice and reduce the level of anti-DNA antibodies and lymphadenopathies in MRL/lpr mice.