The RA3-6B2 antibody reacts with the human and mouse CD45 isoform known as CD45R, or B220, a protein tyrosine phosphatase of ≥ 220 kDa. CD45 is one of the most abundant hematopoietic markers, and is expressed on all leukocytes (the Leukocyte Common Antigen, LCA). Various isoforms are generated and expressed in cell-specific patterns, all critical for leukocyte function. In mouse, the CD45R/B220 isoform is predominantly found on B cells, at varying levels on all stages from pro-B cells to activated B cells, and may also be detected on certain T cell and NK cell subsets. It is of note that B220 is not similarly expressed on human B cells, where it appears to be differentiation-specific and therefore expressed on only some B cell subsets. Other forms of CD45 with restricted cellular expression include CD45RA, CD45RB, CD45RO and several others.