CD3

by Jason Wasserman MD PhD FRCPC
May 6, 2022

What is CD3?

CD3 is a protein that is normally made by two types of specialized immune cells: T cells and NK cells. Most lymphomas that start from T and NK cells, including peripheral T cell lymphoma, anaplastic large cell lymphoma, and NK/T cell lymphoma, also make CD3.

Why do pathologists test for CD3?

The most common reason that pathologists test for CD3 is to find out if the cells they are seeing under the microscope are T or NK cells. This is especially important when examining a tumour. If all, or most, of the tumour cells are making CD3, it is more likely that the tumour is a type of lymphoma made up of T or NK 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 CD3-producing T cells when examining a tissue sample for celiac disease.

How do pathologists test for CD3?

Two common tests used to look for CD3 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 a microscope. Flow cytometry uses a special machine to count and analyze the number of cells in a tissue sample that were making CD3.

What does “positive for CD3” mean?

“Positive for CD3” means that the cells of interest in the tissue sample were producing this protein. Both normal and cancerous T and NK cells will be positive for CD3.

What does “negative for CD3” mean?

“Negative for CD3” means that the cells of interest in the tissue sample were not producing this protein. Other than T and NK cells, most types of cells will be negative for CD3.

What’s next?

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 or flow cytometry tests before making a final diagnosis.