CD79a: Definition

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CD79a is a protein found in B cells, a type of white blood cell that makes antibodies. It forms part of the receptor that B cells use to recognize germs. Pathologists test for CD79a to confirm that the cells in a lymphoma or leukemia are B cells. It is especially useful in tumors that have lost other B-cell markers.

CD79a is a test result, not a diagnosis. It is one of the stains a pathologist uses to work out what type of cells are present. This article explains why pathologists test for CD79a and what a positive or negative result means in your pathology report.

Where is CD79a normally found?

CD79a appears early in a B cell’s life and remains present as the cell matures. It is found in very young B cells in the bone marrow and in mature B cells in the lymph nodes. It is also found in plasma cells, which are fully mature B cells.

This range is wider than that of CD20, the most commonly used B-cell marker. CD20 is usually missing from the youngest B cells and from plasma cells.

How do pathologists test for CD79a?

Pathologists test for CD79a using immunohistochemistry (IHC). In this test, a thin slice of tissue on a glass slide is treated with an antibody that sticks to CD79a. A chemical reaction then colors any cell containing CD79a brown so that it can be seen under the microscope.

Pathologists can also identify B cells by flow cytometry, a test that examines cells in a liquid sample such as blood or bone marrow.

Why do pathologists test for CD79a?

Pathologists use CD79a to confirm that cells are B cells. It does not predict how a tumor will respond to a particular treatment. Pathologists test for CD79a in the following situations:

To identify B-cell lymphomas. Most B-cell lymphomas, such as diffuse large B-cell lymphoma and follicular lymphoma, are CD79a positive. It is usually tested along with CD20 and PAX5.
To identify B-cell tumors that have lost CD20. Some lymphomas lose CD20, particularly after treatment with medicines that target CD20. CD79a can still show that the tumor is made of B cells.
To identify young B-cell tumors. B-lymphoblastic leukemia and lymphoma are made of very young B cells that often lack CD20 but contain CD79a.
To identify plasma cell tumors. Plasma cell tumors, such as plasmacytoma and plasmablastic lymphoma, are often positive for CD79a but negative for CD20.

What does a positive CD79a result mean?

A positive result means the tested cells contain CD79a. Your report may describe the result as “positive” or say that CD79a “is expressed.” A positive result supports that the cells are B cells or plasma cells.

Rarely, other tumor types, including some T-cell and myeloid leukemias, can be positive for CD79a. For this reason, the pathologist interprets the result together with other markers.

What does a negative CD79a result mean?

A negative result means the tested cells do not contain CD79a. Your report may describe the result as “negative” or “not expressed.” In a lymphoma, a negative result makes a B-cell tumor less likely and supports another type, such as a T-cell lymphoma. Normal B cells in the same sample are usually still positive, which confirms that the test worked.

Questions to ask your doctor

Why was CD79a tested on my sample?
Was the CD79a result positive or negative?
Is my lymphoma a B-cell or a T-cell type?
Was CD20 also tested, and what did it show?
Does the CD79a result affect my treatment options?
What is the final diagnosis in my report?