Flow cytometry is a diagnostic technique that is used to measure the chemical or physical characteristics of cells in suspension. Flow cytometry can also determine the types and the quantities of antigens expressed on cell membranes through a process called immunophenotyping. Antigens are substances that are capable of activating the immune system. Each antigen category is called a cluster of differentiation (CD) and is numbered.

This is very much an evolving field. Isolation of CD markers is continuing and new ones are being found monthly. As a result, marker panels will change, and more specific information will be available. As a general rule, it can be said that,

Cells that are positive for CD2, 3, 4, 5, 8, 45RA and/or 45RO are T-lymphocytes.
Cells that are positive for CD10, 19, 20, 21, 23, 35, 40, and sometimes 77 are B-lymphocytes (CD5 can also be found on a specific subset of B-lymphocytes).
CD28 can be positive with T or B-cells and NK (natural killer) cells, which are another lymphocyte subset.
CD34 is one of the most important markers for it is seen in the hematopoietic pluripotential stem cell (PSC) which is the cell that is "wanted" in bone marrow or peripheral blood stem cell transplants.
CD38 is a receptor that is found in plasma cells, some thymocytes (early lymphocytes still in the thymus), NK cells, and in very early B-cells. Increased presence of CD38 has been found in cells in multiple myeloma, and certain acute lymphoblastic and myeloblastic leukemias. CD38 is also thought to be of predictive value in determining the clinical course that CLL will take.

In CLL cells, a distinct pattern of antigens is expressed. CLL lymphocytes co-express the B-cell antigens CD19 and CD20 along with the T-cell antigen CD5. The analysis of a blood sample by flow cytometry is therefore very useful in confirming the diagnosis of CLL.

The following diagram illustrates how CD markers are used in the diagnosis of disorders that are characterized by elevated lymphocyte counts (lymphocytosis). It is presented here with permission of the copyright holder, Dr. Margaret Uthman.

The terms kappa and lambda, which appear on flow cytometry reports, refer to portions of the immunoglobulin or antibody molecule. Kappa and lambda are long chains of amino acids. While there are heavy chains and light chains, kappa and lambda are both light chains. Most people should be able to make either of these light chains in the amounts appropriate for antibody activity. Over expression of one of the chains is often seen in CLL patients and usually means a loss of control by the cells.

When the leukemias were first discovered, the only tool available was the microscope. Now we have learned how to identify cells by their cell membranes. It is hoped that we will soon be able to identify cells by their DNA.

Flow cytometry is very time consuming to perform. Cells cannot be tested for every known antigen--it's too expensive, requires too much time, and there is too great a possibility of error. For these reasons, every lab makes a series of decisions concerning which tests will be run on which samples.

In CLL, malignant lymphocytes derive from cells gone berserk (monoclonal expansion). If we can find out what type of cell it is (for example, does it have a compound on its membrane called CD20), then it is possible to design a drug (monoclonal antibody) to fight only CD20+ cells. This is the underlying principle of medications like rituximab (Rituxan).