The HB220 antibody reacts with mouse CD45RB a 220 kDa transmembrane protein tyrosine phosphatase expressed on peripheral B cells naëâve T cells thymocytes and weakly on macrophages and dendritic cells. CD45RB is one of 5 isoforms of CD45 which result from alternative splicing of exons 4-6. CD45RB results from exon 5 splicing. CD45RB is critical for intracellular signaling in T cells in response to antigen stimulation as indicated by the severe immunodeficiency observed in CD45-deficient mice. As T cells progress from naëâve to memory cells and become activated CD45RB expression is downregulated. Additionally the intensity of CD45RB expression can be used to differentiate between functionally distinct CD4⁺ T cell subsets which secrete differing cytokine profiles. The primary ligands for CD45 include galectin-1 CD2 CD3 CD4 and Thy-1.