CD45RB and CD45RAB are specific isoforms of CD45, a receptor-type protein tyrosine phosphatase (PTP) found on the surface of all nucleated hematopoietic cells, except mature erythrocytes and platelets. They serve as essential phenotypic markers to distinguish naïve T lymphocytes from antigen-experienced memory/effector T cells. CD45RB and CD45RAB designations refer to specific isoforms of CD45 generated through alternative splicing of three variable exons (4, 5, and 6) in the PTPRC gene. These exons encode extracellular domains A, B, and C, respectively, which contain O-linked glycosylation sites that affect the molecule's size, charge, and ligand-binding properties. This antibody can detect CD45RB (contains exons B only) and CD45RAB (contains both exons A and B), but not CD45RABC (contain all three exons) or CD45RBC (contain exons B and C).