by Jason Wasserman MD PhD FRCPC
May 6, 2022

What is CD20?

CD20 is a protein that is normally made by specialized immune cells called B cells. B cells are found throughout the body but the greatest number are found in immune organs such as lymph nodes.  Most lymphomas that start from B cells, including chronic lymphocytic leukemia (CLL), small lymphocytic lymphoma (SLL), follicular lymphoma, and mantle cell lymphoma also make CD20.

Why do pathologists test for CD20?

The most common reason that pathologists test for CD20 is to find out if the cells they are seeing under the microscope are B cells. This is especially important when examining a tumour. If all (or most) of the tumour cells are making CD20, it is more likely that the tumour is a type of lymphoma made up of B cells. It is normal to see a large number of B cells in some types of tissue such as lymph nodes, the oropharynx (tonsils, base of tongue, and uvula), and the digestive tract.

How do pathologists test for CD20?

Two common tests used to look for CD20 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 CD20.

What does “positive for CD20” mean?

“Positive for CD20” means that the cells of interest in the tissue sample were producing this protein. Both normal and cancerous B cells are typically positive for CD20. Other B cell markers include CD19, CD79, and PAX-5.

What does “negative for CD20” mean?

“Negative for CD20” means that the cells in the tissue sample were not producing this protein. Other than B cells, most types of cells will be negative for CD20.

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.

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