CD45, also known as protein tyrosine phosphatase receptor type C (PTPRC), is a crucial transmembrane glycoprotein expressed on the surface of all nucleated hematopoietic cells except erythrocytes and plasma cells. It is a receptor-like protein tyrosine phosphatase that plays a vital role in regulating immune cell activation and differentiation. CD45 consists of an extracellular domain, a transmembrane segment, and intracellular phosphatase domains. Its primary function is to dephosphorylate specific tyrosine residues on target proteins, thereby modulating signaling pathways essential for immune responses. CD45 has multiple isoforms generated by alternative splicing, which are differentially expressed across immune cell subsets and influence their activation thresholds. For example, na茂ve T cells predominantly express CD45RA, while memory T cells express CD45RO, which enhances T-cell receptor signaling. In pathology, CD45 is used as a marker to identify hematopoietic cells and distinguish lymphomas from other types of cancer.