CD45

CD45, the leukocyte common antigen, is a receptor-type protein tyrosine phosphatase encoded by the PTPRC gene and expressed on all nucleated hematopoietic cells. Its large extracellular region is subject to alternative splicing that yields isoforms such as CD45RA on naive T cells and CD45RO on memory T cells, while cytoplasmic phosphatase domains dephosphorylate inhibitory tyrosines of the Src-family kinases Lck and Fyn, setting the threshold for T- and B-cell antigen receptor signaling. Loss-of-function PTPRC mutations cause severe combined immunodeficiency. Because isoform usage changes with lymphocyte differentiation, CD45 staining with lineage markers underpins flow-cytometric leukemia diagnosis and immunophenotyping panels.