To be “CD45 positive” means that a specific protein, known as CD45, is present on the surface of a cell. Think of this protein as a uniform worn exclusively by a particular team of cells within the body. When laboratory tests detect this protein on a cell, that cell is labeled CD45 positive. The absence of this protein makes a cell CD45 negative, a distinction used to identify and sort different types of cells, particularly those of the immune system.
The Function of the CD45 Protein
The CD45 protein is more than just a surface marker; it functions as a regulator for immune cells. Scientifically, it is a protein tyrosine phosphatase, an enzyme that modifies other proteins to control cellular activities. It acts like a molecular switch that helps to manage the activation of immune cells, ensuring these cells respond appropriately to threats.
When an immune cell encounters a foreign invader, such as a virus or bacteria, CD45 helps to transmit the “go” signal, initiating the immune response. Conversely, this protein is also involved in telling the immune cell when to stand down after the threat has been neutralized. This prevents the immune system from overreacting and causing damage to the body’s own tissues.
Which Cells are CD45 Positive?
The CD45 protein is a hallmark of leukocytes, the scientific term for white blood cells. Its presence is a defining characteristic of nearly all cells that make up the immune system. This includes the various families of leukocytes that each have specialized jobs in defending the body from infection and disease.
Specifically, lymphocytes, which include both T-cells and B-cells, are CD45 positive. Monocytes and macrophages, which engulf and digest cellular debris and pathogens, also express this marker. Granulocytes, a category that includes neutrophils, eosinophils, and basophils, are all part of the CD45 positive family as well.
In contrast, most other cells in the human body are CD45 negative. This includes red blood cells, which are responsible for oxygen transport, and platelets, which are involved in blood clotting. Non-hematopoietic cells, meaning cells not derived from the blood-forming tissues in the bone marrow, also lack this protein. This group encompasses the cells that form organs and tissues, such as skin cells, muscle cells, and nerve cells.
Clinical Relevance of CD45 Testing
The specific presence of CD45 on white blood cells makes it a useful tool in clinical diagnostics. Pathologists use it as a “pan-leukocyte marker,” meaning a universal identifier for all white blood cells. This allows them to distinguish immune cells from other cell types within a tissue sample, which is useful for diagnosing a wide range of conditions.
When a patient has a tumor, determining the origin of the cancerous cells is a primary step. Cancers that arise from immune cells, such as lymphomas and leukemias, are composed of CD45 positive cells. In contrast, most other types of tumors, such as carcinomas or sarcomas, are made of CD45 negative cells, a distinction that helps pathologists arrive at a precise diagnosis.
Beyond cancer diagnosis, CD45 testing has other applications. For patients who have received a bone marrow transplant, monitoring the levels of CD45 positive cells in their blood tracks the success of the procedure. A healthy rise in these cells indicates that the transplanted stem cells are successfully generating a new, functioning immune system.
Interpreting CD45 Test Results
CD45 levels are measured using specialized laboratory techniques that can detect the protein on individual cells. The two primary methods are flow cytometry and immunohistochemistry. Flow cytometry is used for liquid samples like blood or bone marrow, where cells are suspended in a fluid and passed through a laser-based detector that identifies how many are CD45 positive.
Immunohistochemistry is applied to solid tissue samples, such as a biopsy from a lymph node or other organ. In this technique, a tissue slice is treated with antibodies that bind specifically to the CD45 protein. A chemical reaction then makes the positive cells visible under a microscope.
A blood sample with a very high count of CD45 positive cells could suggest the body is fighting a severe infection. It could also be an indicator of leukemia, a cancer characterized by the overproduction of white blood cells. The specific amount and intensity of CD45 expression can provide further clues.
When a biopsy is taken from a non-lymphoid organ, such as the lung or liver, the presence of CD45 positive cells is evaluated. While a small number of these cells are normal, as the immune system patrols all tissues, a large collection of CD45 positive cells could confirm a diagnosis of lymphoma that has spread to that organ. It can also indicate a site of significant, chronic inflammation.