Understanding the CD20 Marker in Lymphoma

What is CD20 and How Does it Help in Lymphoma Treatment?

Ross Hutchins receives treatment for Hodgkin's lymphoma. Bryn Lennon / GettyImages

Definition: CD20 Tumor Marker

CD20 is a CD marker — a molecule on the cell surface that can be used to identify and type a particular cell in the body. CD20 is found on the surface of B cells, but lets back up and make this easier to understand.

What are CD Markers?

CD markers are molecules that are found on the surface of cells in our bodies. You may hear them called antigens — and an antigen is basically anything on the surface of a cell that can be recognized by our immune system.

For example, our white blood cells may recognize antigens on the surface of bacterial invaders and the white cells have the ability to respond to what is correctly perceived as a threat.

Every cell in our bodies has a CD marker, and altogether there are more than 250 of these antigens. The term CD stands for cluster of differentiation - and again, is one way in which different kinds of cells can be told apart. Being able to identify these CD markers can be especially important when you cannot otherwise recognize the type of cell.

CD Markers and B Cells and T Cells

Lymphomas are cancers of the type of white blood cell known as lymphocytes. There are two main types of lymphocytes - B lymphocytes or B cells, and T lymphocytes or T cells. Both types of lymphocytes help to protect our bodies from infections.

B cells and T cells have different functions, and cancers of each of the B cells act differently from cancers of the B cells.

Despite all of these differences, it can be difficult to tell the difference between the 2 under a microscope. Kind of like 2 identical twins who behave very differently and respond to people differently but look alike on the outside.

What Exactly is CD20?

CD20 is an antigen that is found on the surface of B cells but not T cells.

An example of how CD-20 can be used to tell cells apart is given here:

There are 2 very different cancers that look like identical twins under the microscope, but have very different disease courses and respond differently to treatments: Diffuse large B cell lymphoma (DLBCL) is a cancer involving B cells. Under the microscope the cells look just like the cancerous T cells found in anaplastic large cell lymphoma (ALCL). Both of these cancers have large, "cancery" looking cells and can be otherwise indistinguishable. CD20 can be used to tell the difference between these two cancers in that test results for CD20 would usually be positive in the case of DLBCL but negative for ALCL.

How is it Tested?

A special technique called immunohistochemistry (IHC) is used to identify CD20 and determine whether an abnormal cancerous white blood cell (lymphocyte in particular) is a B-cell or T-cell.

What Does it Mean for Treatment?

Treatment and prognosis for B-cell and T-cell lymphomas are often different.

A newer category of medications called monoclonal antibodies work very well for some lymphomas.  Just as our bodies make antibodies to fight off bacteria and viruses, monoclonal antibodies are man made antibodies designed to fight off cancer cells.

And just as our bodies make antibodies that recognize antigens on bacteria an viruses, these monoclonal antibodies recognize antigens on the surface of cancer cells; in this case, CD20.

Using a monoclonal antibody that binds to CD20 will therefore only work against a cancer with CD20 antigens on the surface, such as ALCL in the analogy above.

Monoclonal Antibody Treatment and CD20

There are several monoclonal antibodies that are now being used. The monoclonal antibodies that treat B cell lymphomas and leukemias with the CD20 antigen on the surface of the cells include:

  • Bexxar (tositumomab)
  • Gazyva (obinutuzumab)
  • Arzerra (ofatumumab)

Though they all bind CD20, there are differences among these antibodies. Some are referred to as chimeric such as rituximab, which means the bio-engineers used a 'mix of construction materials' to craft the antibody, some human, some mouse; some are humanized (obinutuzumab) and some are fully human (ofatumumab), meaning all parts come from human gene => protein sources. Then, another differentiating factor is that some are linked to a radioactive elements (ibritumomab tiuxetan and tositumomab).

Coping with Lymphoma

As you've reached this page, you are taking a great step forward in being an active participant in your care. Studies have found that learning as much about your disease as possible, and playing an active role in your care, helps not only cope with the anxiety of cancer, but may help with outcomes as well.  Reach out to family and friends. Check into the wonderful online community of people with leukemia and lymphoma - people you can access 24/7 for support and with questions. And make sure you are your own advocate in your cancer care. Medicine is changing and oncologists not only tolerate but expect to work side by side with you to tailor a treatment program that is best for you as a person.

Sources:

National Cancer Institute. Adult Non-Hodgkin Lymphoma Treatment – Health Professional Version (PDQ). Updated 01/15/16. http://www.cancer.gov/types/lymphoma/hp/adult-nhl-treatment-pdq

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