The RA3-6B2 monoclonal antibody specifically reacts with a 220 kDa exon A-restricted epitope on the extracellular domain of mouse CD45 glycoprotein. CD45R/B220 is expressed on all the development stages of B cells, and at a low level on plasma cells and a subset of memory B cells. Low levels of CD45R/B220 are also expressed by some activated T lymphocytes, natural killer cell progenitors in the bone marrow, lymphokine activated killer cells (LAK), T lymphocytes of lpr/lpr mutant mouse, apoptotic T cells of mice injected with bacterial superantigens, and macrophage progenitors in fetal liver.The CD45 molecule is a member of the Protein Tyrosine Phosphatase (PTP) family, because its intracellular region contains two PTP domains. The extracellular region’s variability is caused by different levels of glycosylation, and the splicing of the 4, 5, and 6 exons. The isoforms found in the mouse strains depend on the activation state, maturation stage and cell type, and are very important in B and T lymphocytes antigen receptor signal transduction.The RA3-6B2 antibody inhibits in vivo B lymphocytes responses and enhances isotype switching during in vitro B lymphocyte responses.