Lymphocyte Typing

Definition

Lymphocyte typing focuses on identifying the numbers and relative percentages of lymphocytes in an individual's bloodstream. Lymphocytes, primarily T cells and B cells, are types of white blood cells, the underlying supports of the immune system in the bloodstream.

Purpose

Determining the numbers and relative percentages of T cells and B cells provides information on the state of a person's immune system. By comparing these values to normal numbers and percentages, the presence of disease and the side effects of certain drugs can be revealed. Lymphocyte typing can also show whether a person has been exposed to certain poisonous substances.

Description

To do a white blood cell count, a small amount of blood is drawn from a vein. The total number of white blood cells is calculated, either through microscopic examination of a blood smear or by using automated counting equipment. For a white blood cell count with differential, 100 white blood cells are counted and the proportion of each type is calculated. Since T cells and B cells have similar appearances, a differential can only give the proportion of lymphocytes in the blood, not the proportion of specific lymphocyte types.

For more specific information on B cells and T cells, it is necessary to divide the blood into its separate components. In this procedure, a tube of blood is placed in a centrifuge, a piece of equipment that spins the tube in circles at high speed. The force generated by the spinning causes the various elements in the bloodstream to settle at different levels of the tube.

The lymphocytes are extracted from the tube and treated with special dyes, or stains. Each stain is equipped with an antibody portion that adheres to a specific type of lymphocyte, such as a B cell or a T cell. The stains make the cells visible to an automated counting machine, called a flow cytometer. Based on the number of times the machine detects a particular stain, it can calculate the number of the associated cell type. This procedure can also be used to classify T cells and B cells into their subtypes.

Preparation

If possible, a person should avoid eating a heavy meal within hours of the test or engaging in strenuous exercise for the 24 hours preceding the blood test.

Normal results

In general, normal levels of white blood cells vary slightly by age and gender. Normal values are lower in children under the age of 15 and in young adults between the ages of 20 and 30. After age 30, men have slightly higher levels of white blood cells than women.

Normal adult levels of white blood cells are 4,500-11,000 cells per microliter of blood. Lymphocytes account for approximately 25-45% of the total white blood cell count; the normal range is 1,000-4,800 lymphocytes per microliter of blood. Of the total lymphocytes, 60-80% are T cells and approximately 15% are B cells. (There are two other types of lymphocytes; natural killer and K-type; that constitute a minor proportion of the total lymphocyte numbers.)

Abnormal results

A higher-than-normal level of lymphocytes is called lymphocytosis. Lymphocytosis occurs if a person has a viral, bacterial, or other type of infection. It can also occur with certain blood disorders, such as leukemia.

Lower-than-normal levels of lymphocytes is called lymphopenia. Lymphopenia can be an indicator of certain cancers, bone marrow failure, or immune system deficiency. Medical treatments, such as chemotherapy and radiation therapy, can also deplete the body's supply of lymphocytes, as can exposure to poisonous substances.

BOOKS

Corbett, Jane Vincent. Laboratory Tests & Diagnostic Procedures with Nursing Diagnoses. 4th ed. Stamford: Appleton & Lange, 1996.

Janeway, Charles A., and Paul Travers. Immunobiology: The Immune System in Health and Disease. 2nd ed. New York: Garland Publishing, Inc., 1996.

Malarkey, Louise M., and Mary Ellen McMorrow. Nurse's Manual of Laboratory Tests and Diagnostic Procedures. Philadelphia: W. B. Saunders Co., 1996.

Turgeon, Mary Louise. Immunology & Serology in Laboratory Medicine. St. Louis: Mosby-Year Book, Inc., 1996.

Julia Barrett

KEY TERMS

Immune system—The body's system of defenses against infectious diseases.

Lymphocytosis—A condition in which the number of lymphocytes increases above normal levels.

Lymphopenia—A condition in which the number of lymphocytes falls below normal levels.

White blood cell—A class of cells in the blood that form the foundation of the body's immune system.