CD45 is a glycoprotein with protein phosphatase activity abundantly expressed on the surface of all leukocytes. The protein is highly preserved during evolution and has been demonstrated even in chicken, sharks, and mosquitos. CD45 increases T-cell and B-cell receptor signaling as well as the development and function of leukocytes. Moreover, CD45 can reduce Toll-like receptors and cytokine signaling, as well as cell adhesion and migration in a cell type specific manner. In humans, six splice variants differing in the extracellular part of the molecule have been demonstrated. The human variants are termed CD45RABC (also named CD45R), CD45RAB, CD45RBC, CD45RA, CD45RB, and CD45RO, and are detected by subtype specific antibodies and molecular weight, respectively. CD45RO is expressed on most thymocytes, activated T cells, memory T cells, granulocytes and monocytes, but only on a proportion of resting T cells.