CD45, also known as Protein Tyrosine Phosphatase Receptor Type C (PTPRC), is a member of the protein tyrosine phosphatase (PTP) family. It is a type I transmembrane protein expressed on the surface of all differentiated hematopoietic cells, except erythrocytes and plasma cells. CD45 plays a crucial role in regulating T-cell and B-cell antigen receptor signaling, which is essential for immune cell activation and function. CD45 consists of an extracellular domain, a single transmembrane segment, and two tandem intracytoplasmic catalytic domains. While only one of the cytoplasmic domains has intrinsic phosphatase activity, both domains are necessary for substrate recruitment. CD45 activates Src family kinases, such as Lck, which are required for T-cell receptor (TCR) signaling. It also modulates the activity of other kinases, including LYN and FYN, and suppresses JAK kinases, thereby acting as a negative regulator of cytokine receptor signaling. CD45 is involved in various cellular processes, including cell growth, differentiation, and immune response regulation. Dysregulation of CD45 function can lead to severe combined immunodeficiency and is implicated in autoimmune diseases, cancer, and infectious diseases. Additionally, CD45 exists in multiple isoforms, which are used in immunohistochemistry to differentiate between immune cell types and diagnose lymphomas and carcinomas.