CD45 is a type I transmembrane protein that is present in various isoforms on all differentiated hematopoietic cells （except erythrocytes and plasma cells）. CD45 has been shown to be an essential regulator of T- and B-cell antigen receptor signaling. It functions through either direct interaction with components of the antigen receptor complexes via its extracellular domain （a form of co-stimulation）, or by activating various Src family kinases required for the antigen receptor signaling via its cytoplasmic domain. CD45 also suppresses JAK kinases, and so functions as a negative regulator of cytokine receptor signaling.