by Jason Wasserman MD PhD FRCPC
May 6, 2022
CD5 is a protein that is normally made by specialized immune cells called T cells. Most lymphomas that start from T cells, including peripheral T cell lymphoma, anaplastic large cell lymphoma, and extranodal T cell lymphoma, make CD5. Abnormal B cells can also make CD5, and some lymphomas that start from these B cells, such as chronic lymphocytic leukemia (CLL) and mantle cell lymphoma, also make CD5.
The most common reason that pathologists test for CD5 is to see if the cells they see under the microscope are T cells. Pathologists also test for CD5 to look for abnormal B cells, which can also make CD5. This is especially important when examining a tumour. If all, or most, of the tumour cells are making CD5, it is more likely that the tumour is a type of lymphoma made up of T cells or abnormal B cells. It is normal to see an increased number of T cells in the tissue around an infection or in an area of inflammation. These T cells contribute to the normal healing and repair process. However, some medical conditions are also characterized by an increased number of T cells. For example, pathologists often look for an increased number of CD5 producing T cells when examining a tissue sample for celiac disease.
Two common tests used to look for CD5 in a tissue sample are immunohistochemistry and flow cytometry. Immunohistochemistry is performed on a tissue sample attached to a glass slide. The slide is then examined under the microscope. Flow cytometry uses a special machine to count and analyze the number of cells in a tissue sample that are making CD5.
“Positive for CD5” means that the cells of interest in the tissue sample were producing this protein. Both normal and cancerous T cells will usually be positive for CD5. Some lymphomas that start from B cells will also be positive for CD5.
“Negative for CD5” means that the cells of interest in the tissue sample were not producing this protein. T cells that do not produce CD5 are considered abnormal. Other than T cells and some abnormal B cells, most other types of cells will be negative for CD5.
Your pathologist will combine the result of this test with other information such as the microscopic features seen on the routine hematoxylin and eosin (H&E) slide and the results of other immunohistochemistry tests before making a final diagnosis.