CD23: Definition

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CD23 is a protein found on the surface of some B cells and on a type of support cell inside lymph nodes called the follicular dendritic cell. Pathologists most often test for CD23 to help diagnose chronic lymphocytic leukemia and small lymphocytic lymphoma, and to separate them from other small-cell lymphomas. CD23 also helps the pathologist see the normal structure of a lymph node and tell whether a lymphoma has replaced it.

This article explains what CD23 is, which cells carry it, how pathologists use it to diagnose lymphoma, and what a CD23 result in your pathology report means.

What is CD23?

CD23 is a protein that sits on the outer surface of certain immune cells. There, it helps B cells grow, respond to infections, and make antibodies. It acts as a receptor for IgE, the antibody type involved in allergic reactions. In normal tissue, several types of cells carry CD23:

B cells. Many mature B cells, especially those responding to an infection, carry CD23 on their surface.
Follicular dendritic cells. These support cells form a fine web inside the germinal centers of lymph nodes, where they help B cells learn to recognize germs.
Activated macrophages. These immune cells express CD23 when they respond to infection or injury.

How do pathologists test for CD23?

Pathologists test for CD23 using immunohistochemistry, a test that uses antibodies to find specific proteins in a thin slice of tissue. Cells that carry CD23 turn brown under the microscope, usually along the outer edge of the cell. Laboratories can also measure CD23 on blood or bone marrow cells with flow cytometry, a common way to diagnose chronic lymphocytic leukemia.

How does CD23 help diagnose lymphoma?

CD23 helps diagnose lymphoma because different lymphomas made of small B cells show different CD23 results. Chronic lymphocytic leukemia and small lymphocytic lymphoma are the same disease, and the cancer cells are positive for CD23 in almost all cases. The pathologist usually evaluates CD23 along with CD5, CD20, and cyclin D1. Typical patterns include:

Chronic lymphocytic leukemia and small lymphocytic lymphoma. The cells are usually positive for CD5 and CD23 and negative for cyclin D1.
Mantle cell lymphoma. The cells are usually positive for CD5 and cyclin D1 but negative for CD23, although a few cases show weak CD23.
Marginal zone lymphoma. Cells in nodal marginal zone lymphoma and MALT lymphoma are usually negative for both CD5 and CD23.

Distinguishing these lymphomas matters because each grows at a different pace and needs its own plan of care. A single CD23 result never makes the diagnosis by itself, so the pathologist combines it with the other markers and the appearance of the cells.

How does CD23 show the structure of a lymph node?

CD23 shows the structure of a lymph node by highlighting the web of follicular dendritic cells inside each follicle. In a normal or reactive lymph node, this web appears as neat, round patches. In some lymphomas, the pattern changes in ways that help the pathologist make a diagnosis:

Follicular lymphoma. The web of follicular dendritic cells outlines the crowded, abnormal follicles that give this lymphoma its name.
Marginal zone lymphoma. The web often looks broken up because the lymphoma cells have grown into and around the follicles.
Follicular dendritic cell sarcoma. In this rare cancer, the tumor cells themselves are follicular dendritic cells, so they are positive for CD23.

Where else do pathologists use CD23?

Pathologists also use CD23 to help diagnose primary mediastinal large B-cell lymphoma, a fast-growing lymphoma that starts in the center of the chest. Many of these lymphomas are CD23-positive, which helps separate them from other types of diffuse large B-cell lymphoma. CD23 is also positive in some cases of follicular lymphoma that grow in a diffuse sheet rather than in follicles.

What does a CD23 result mean in my report?

A CD23 result in your report describes whether the cells being studied carry this protein. Your report may describe the result in one of several ways:

Positive. The abnormal cells carry CD23, which supports diagnoses such as chronic lymphocytic leukemia or small lymphocytic lymphoma when the other markers fit.
Negative. The abnormal cells do not carry CD23, which points toward other lymphomas such as mantle cell lymphoma or marginal zone lymphoma.
Weak or dim. The cells carry only a small amount of CD23, which the pathologist interprets alongside cyclin D1 and other markers.
Highlights follicular dendritic cells. The stain shows the web of support cells in the lymph node rather than the lymphoma cells themselves.

Questions to ask your doctor

Were the abnormal cells in my sample positive or negative for CD23?
Which other markers did the pathologist test along with CD23?
What type of lymphoma or leukemia do I have?
Was flow cytometry done on my blood or bone marrow?
Did the CD23 stain show the normal structure of my lymph node?
Will I need more tests, such as genetic tests, to confirm the diagnosis?
What does this diagnosis mean for my follow-up and care?