CD45

CD45, also known as the leukocyte common antigen and formally as protein tyrosine phosphatase receptor type C (PTPRC), is a transmembrane protein tyrosine phosphatase expressed on most nucleated hematopoietic cells. It serves as a major marker of leukocytes in immunophenotyping. Erythrocytes and platelets do not express CD45.

The extracellular portion of CD45 is heavily glycosylated and contains a mucin-like region along with fibronectin type III–like domains. It has a single transmembrane segment and an intracellular protein tyrosine phosphatase (PTP) catalytic domain. Alternative splicing of the PTPRC transcript generates several CD45 isoforms, including CD45RA, CD45RB, CD45RC, and CD45RO. In humans, naive T cells typically express CD45RA, whereas memory T cells often express CD45RO.

Functionally, CD45 regulates antigen receptor signaling in lymphocytes by dephosphorylating inhibitory tyrosine residues on Src family kinases, such as Lck in T cells and Lyn in B cells. This modulation helps set activation thresholds for TCR and BCR signaling, influencing lymphocyte development, activation, and function. CD45 is essential for proper immune cell signaling; deficiency in model organisms disrupts lymphocyte development and signaling.

clinically, CD45 is widely used as a pan-leukocyte marker in flow cytometry to identify and quantify white blood cells and to characterize lymphocyte subsets. Isoform expression patterns assist in identifying differentiation states of T cells. Altered CD45 expression or PTPRC mutations have been linked to immune dysregulation and, in some cases, immunodeficiency or hematologic malignancies, underscoring its role in immune biology.