CD45 positive cells are a broad category of immune cells, also known as leukocytes or white blood cells. They identify and neutralize foreign invaders like bacteria and viruses, and abnormal cells. Their presence is fundamental to a functioning immune response.
The CD45 Protein Marker
A cell surface marker identifies a cell’s specific type or role, similar to how different uniforms distinguish players on various sports teams. CD45 is a protein found on the outer surface of immune cells. The term “CD” stands for “Cluster of Differentiation,” a system used to classify proteins on cell surfaces, helping categorize different cell types.
CD45 functions as an enzyme, specifically a protein tyrosine phosphatase (PTP). It helps regulate how immune cells become activated and communicate by adding or removing phosphate groups from other proteins. This enzymatic activity controls cell growth, differentiation, and the cell cycle.
Types of CD45 Positive Cells
CD45 is found on nearly all nucleated cells from the hematopoietic lineage, which develop into various blood components. This includes major categories of leukocytes, each with distinct roles in the immune system. Lymphocytes, such as T-cells and B-cells, are responsible for targeted immune responses, recognizing specific pathogens and remembering them. T-cells can directly kill infected cells or help coordinate other immune cells, while B-cells produce antibodies to neutralize threats.
Monocytes circulate in the blood and differentiate into macrophages in tissues, which engulf and digest cellular debris, foreign substances, and pathogens. Granulocytes, including neutrophils, eosinophils, and basophils, are also CD45 positive. Neutrophils are the first responders to infection, engulfing microorganisms. Eosinophils are involved in allergic reactions and fighting parasites, while basophils contribute to allergic responses by releasing histamine.
In contrast, red blood cells, which transport oxygen, and platelets, involved in blood clotting, do not typically express CD45. Most non-immune tissue cells, like epithelial or muscle cells, also lack the CD45 marker. This distinction makes CD45 a valuable tool for identifying immune cells within mixed cell populations or tissue samples.
Identifying and Counting CD45 Positive Cells
Scientists and medical professionals use specialized laboratory techniques to detect and quantify CD45 positive cells. Two primary methods are flow cytometry and immunohistochemistry. These techniques allow for the precise identification of immune cells, distinguishing them from other cell types in a sample.
Flow cytometry analyzes cells as they flow in a single file past a laser. Cells from a fluid sample, such as blood or bone marrow, are tagged with specific antibodies that have fluorescent dyes. If a cell has the CD45 protein, the anti-CD45 antibody binds, and the attached dye lights up when hit by the laser. The machine then detects these signals, counting the CD45 positive cells.
Immunohistochemistry (IHC) is another method, often used when examining tissue samples rather than fluid. For IHC, a thin slice of tissue is prepared on a microscope slide. Antibodies designed to specifically bind to the CD45 protein are applied to the tissue. A chemical reaction then occurs, causing the cells where the antibody has bound to change color, making them visible under a microscope. This technique allows pathologists to see not only which cells are CD45 positive but also their distribution and location within the tissue structure.
Clinical Relevance in Diagnosis and Monitoring
The presence or absence of CD45, along with its expression levels, provides important information for diagnosing and monitoring various medical conditions, especially those affecting the immune system. Doctors frequently use CD45 testing as a diagnostic aid to understand cell origins and disease progression.
In the diagnosis of blood cancers like leukemia and lymphoma, CD45 testing is particularly informative. An abnormally high number of specific CD45 positive cells, or unusual patterns of CD45 expression, can indicate the presence of these cancers. For example, flow cytometry can identify populations of leukemic blast cells that often show different CD45 expression levels compared to normal immune cells, aiding in the diagnosis of acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL).
CD45 also plays a role in differentiating tumor types, which helps guide treatment decisions. When a biopsy is taken from a tumor, staining for CD45 can help determine if the tumor originated from immune cells or other tissue types. If a tumor stains positive for CD45, it suggests a lymphoma, a cancer of lymphocytes. Conversely, if the tumor is CD45 negative but expresses other markers like keratins, it indicates a carcinoma, a cancer originating from epithelial cells. This distinction is important for accurate diagnosis and selecting the most effective therapy.
Monitoring immune health is another significant application of CD45 assessment. In patients undergoing procedures such as bone marrow transplants, tracking CD45 positive cell counts helps evaluate the recovery and reconstitution of the immune system. Changes in CD45 expression patterns can also provide insights into the status of immunodeficiency diseases or indicate immune system activation in inflammatory conditions. For instance, the proportions of specific CD45 positive T-cell subsets can be monitored to assess immune suppression or activation in various contexts, including responses to cancer immunotherapy.