T cells are a type of white blood cells called lymphocytes. They make up part of the immune system. T cells help the body fight diseases or harmful substances.

A test can be done to measure the number of T cells in your blood.

Thymus derived lymphocyte count; T-lymphocyte count

A blood sample is needed. For information on how this is done, see: Venipuncture

In the laboratory, the white blood cells (including T cells) are separated from the other blood cells. A stain or other substance that "labels" the cells is added to the sample to help identify which type of white blood cells are present.

No special preparation is necessary.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Your doctor may order this test if you have signs of an immunodeficiency disorder or a disease of the lymph nodes. It is also used to monitor how well therapy for these types of diseases is working.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.

Higher than normal T-cell levels may be due to:

• Acute lymphocytic leukemia
• Babesiosis
• Hepatitis
• Infectious mononucleosis
• Multiple myeloma
• Toxoplasmosis

Lower than normal T-cell levels may be due to:

• AIDS
• Acute viral infection
• Aging
• Cancer
• Congenital T-cell deficiency (rare)
• Hodgkin's disease
• Immunodeficiency disorders
• HIV infection
• Leukemias
• Radiation therapy
• Steroid treatment
• Waldenstrom's macroglobulinemia

Risks associated with having blood drawn are slight:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Note: This test is often performed on people with altered immune systems. Therefore, the risk for infection may be somewhat greater than when blood is drawn from a person with a normal immune system.

This following can affect test results:

• Chemotherapy medications
• Corticosteroids
• Immunosuppressive medications
• Radiation therapy
• Stress
• Surgery

Berliner N. Leukocytosis and leukopenia. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier;2011:chap 170.