CD45, also known as the leukocyte common antigen (LCA), exists as four different isoforms (180, 190, 205, 220 kDa) that are generated by alternative splicing of three exons. LCA is found on all cells of hematopoietic origin, except erythrocytes and platelets. CD45 expression is required for T cell and B cell antigen receptor-mediated activation.1-4