A cell designated as CD45 positive is a member of the body’s immune system. CD45, also known as the Leukocyte Common Antigen (LCA), is a large protein found exclusively on the surface of virtually all white blood cells, or leukocytes. This surface marker acts as a definitive biological label, distinguishing immune cells from all other cell types in the body, such as skin, nerve, or muscle cells. The protein is encoded by the PTPRC gene and is a transmembrane molecule extending from the cell’s exterior surface into its interior cytoplasm.
The Role of CD45 in Immune Cell Signaling
CD45 functions as a powerful regulator of immune cell activity, operating as a protein tyrosine phosphatase (PTP) enzyme. Its primary job is to remove phosphate groups from specific tyrosine amino acids on other proteins inside the cell. By removing these phosphate groups, CD45 acts like a molecular switch, controlling the flow of information that determines whether an immune cell should remain quiet or initiate a response.
This enzymatic activity is significant in T cells and B cells, where CD45 controls the signaling cascades initiated by their respective antigen receptors. In T cells, CD45 activates Src family kinases, such as Lck, which are required to start signal transduction after an antigen is recognized. CD45 also plays a negative regulatory role, helping to suppress weak or accidental signaling events. This dual function ensures that T cells only launch a full-scale immune response when they encounter a genuinely threatening antigen.
Identifying Cell Types Using CD45 Expression
The expression of CD45 serves as the most reliable identifier for cells of hematopoietic origin, meaning those cells that develop in the bone marrow. CD45 positivity is shared by all major types of white blood cells, including lymphocytes (T cells, B cells, and Natural Killer cells), monocytes, macrophages, and granulocytes (neutrophils, eosinophils, and basophils). The strength of CD45 expression varies among these different cell types, aiding in their initial classification.
In contrast, any cell type not part of the immune system is classified as CD45 negative. This includes structural cells like fibroblasts, epithelial cells, muscle cells, and nerve cells. The only exceptions within the blood lineage that lack CD45 are mature red blood cells and platelets, which are non-nucleated or fragmented cells.
CD45 in Medical Diagnosis and Monitoring
The presence or absence of CD45 is a key test in medical diagnostics, especially in hematology and oncology. CD45 expression is commonly analyzed using flow cytometry, a technique that rapidly measures the presence and quantity of surface proteins on thousands of individual cells. This application is used for classifying cancers: a CD45 positive tumor is confirmed as a hematopoietic malignancy, such as leukemia or lymphoma, distinguishing it from CD45 negative solid tumors like carcinomas or sarcomas.
CD45 Isoforms and T Cell Status
Beyond its role as a general immune cell marker, variations in the CD45 protein, known as isoforms, categorize the maturity and history of T lymphocytes. For example, the CD45RA isoform is found on naïve T cells, which have not yet encountered an antigen. Conversely, the CD45RO isoform is expressed on memory T cells that have been previously activated and retain the ability to launch a quicker response upon re-exposure.
Clinicians use these CD45 isoform patterns to monitor the status of a patient’s adaptive immune system. This includes tracking disease progression in autoimmune conditions or assessing immune reconstitution after bone marrow transplantation. The intensity of CD45 expression, when plotted against other cellular characteristics, also helps separate white blood cell populations into distinct clusters for accurate counting and analysis. This analysis is essential for identifying abnormal or malignant cell populations and guiding targeted treatment strategies.