CD45Antigens

CD45 antigens, also known as leukocyte common antigen, are a family of receptor-type protein tyrosine phosphatases expressed on all nucleated cells of the hematopoietic lineage, with high expression on leukocytes. In humans they are encoded by the PTPRC gene.

CD45 proteins have a large extracellular region, a single transmembrane segment, and a cytoplasmic tail containing two dual-specificity phosphatase domains. The extracellular domain includes a mucin-like region and fibronectin type III–like repeats. The cytoplasmic phosphatase domains are arranged so that the proximal D1 domain is catalytically active while the distal D2 domain is generally regulatory.

Alternative splicing of exons that encode the A, B, and C regions generates multiple CD45 isoforms, including CD45RA, CD45RB, CD45RC, and CD45RO. Naive T cells typically express CD45RA, while memory T cells commonly express CD45RO; B cells also display characteristic CD45 isoform patterns. The isoform composition influences the cell’s antigen receptor signaling threshold.

Functionally, CD45 acts as a protein tyrosine phosphatase that modulates signaling by dephosphorylating Src family kinases such as Lck in T cells, thereby shaping activation, differentiation, and maturation of lymphocytes. It participates in the formation and regulation of signaling complexes associated with the T- and B-cell receptor pathways and helps set immune response thresholds.

In clinical and research settings, CD45 serves as a pan-leukocyte marker in flow cytometry and immunophenotyping. Differential isoform expression aids in characterizing lymphocyte subsets, and aberrations in CD45 expression or splicing patterns have been observed in certain immunodeficiencies and hematologic malignancies.