CD45 is a protein tyrosine phosphatase with multiple isoforms differing as a result of alternative splicing of the extracellular domain and glycosylation. CD45 is expressed on all hematopoietic cells except erythrocytes and platelets. CD45RA is one of the CD45 isoforms with the molecular weight of 200-220 kD. It is expressed almost exclusively on B cells. CD45 functions in signal transduction through T and B cell antigen receptors. CD45 has been shown to interact with various proteins, including galectin-1, CD2, CD3, and CD4.