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T cells are a type of white blood cells (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.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

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. 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
• Infectious mononucleosis
• Multiple myeloma

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
• Leukemias
• Side effect of radiation therapy
• 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

Thymus derived lymphocyte count; T-lymphocyte count

Bagby GC. Leucopenia and leukocytosis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 173.

Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.