CD45R, also known as MB1, is an isoform of CD45 that is a member of the protein tyrosine phosphatase (PTPase) family. CD45R is expressed specifically on the surface of hematopoietic cells, and has demonstrated function as a regulator of the antigen and cytokine receptor signalling of B and T cells. Given that the antigen is located in the membrane of all B cells, with the exception of plasma cells and some mature T cells, Anti-CD45R exhibits specific reactivity with most B lymphocytes. The use of Anti-CD45R is primarily useful in distinguishing B cell lymphomas from T cell lymphomas, with specific reactivity to follicle center cells, mantle cells, some medullary thymocytes, and 80% of B cell lymphomas.