lymphocyte

A white blood cell formed in lymphatic tissue throughout the body ( e.g., lymph node s, spleen, thymus, tonsils, Peyer patches, and sometimes in bone marrow) and in normal adults making up approximately 22–28% of the total number of leukocytes in the circulating blood. Lymphocytes are generally small (7–8 μm), but larger forms are frequent (10–20 μm); with Wright (or a similar) stain, the nucleus is deeply colored (purple-blue), and is composed of dense aggregates of chromatin within a sharply defined nuclear membrane; the nucleus is usually round, but may be slightly indented, and is eccentrically situated within a relatively small amount of light blue cytoplasm that ordinarily contains no granules; especially in larger forms, the cytoplasm may be fairly abundant and include several bright red-violet fine granules; in contrast to granules of the myeloid series of cells, those in lymphocytes do not yield a positive oxidase or peroxidase reaction. Lymphocytes are divided into 2 principal groups, termed T and B cells, based on their surface molecules as well as function. Natural killer cells, which are large granular lymphocytes, represent a small percentage of the l. population. SYN: lymph cell, lympholeukocyte. [lympho- + G. kytos, call]

- B l. an immunologically important l. that is not thymus-dependent, is of short life, and resembles the bursa-derived l. of birds in that it is responsible for the production of immunoglobulins, i.e., it is the precursor of the plasma cell and expresses immunoglobulins on its surface but does not release them. It does not play a direct role in cell-mediated immunity. SEE ALSO: T l.. SYN: B cell (2).

- pre-B l. an early B-lymphoid type cell that is recognized by immunofluorescence as a μ-positive, L-chain-negative bone marrow cell.

- Rieder l. an abnormal form of l. that has a greatly indented (or lobed), slightly twisted nucleus; such cells are usually observed in certain examples of chronic lymphocytic leukemia.

- T l. a thymocyte-derived l. of immunologic importance that is long-lived (months to years) and is responsible for cell-mediated immunity. T lymphocytes form rosettes with sheep erythrocytes and, in the presence of transforming agents (mitogens), differentiate and divide. These cells have characteristic CD3 surface markers and may be further divided into subsets according to function, such as helper, cytotoxic, etc. SEE ALSO: B l.. SYN: T cell.

- transformed l. l. transformation.

- tumor-infiltrating lymphocytes (TIL, TILS) (lim′fo-sitz) lymphocytes collected from the site of a tumor and exposed to IL-2 in vitro to expand the population. When these cells are injected back into the tumor-bearing host, they will specifically kill the tumor from which they originated.

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lym·pho·cyte 'lim(p)-fə-.sīt n any of the colorless weakly motile cells that originate from stem cells and differentiate in lymphoid tissue (as of the thymus or bone marrow), that are the typical cellular elements of lymph, that include the cellular mediators of immunity, and that constitute 20 to 30 percent of the white blood cells of normal human blood see B CELL, T CELL

lym·pho·cyt·ic .lim(p)-fə-'sit-ik adj

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n.

a variety of white blood cell (leucocyte), present also in the lymph nodes, spleen, thymus gland, gut wall, and bone marrow. With Romanowsky stains, lymphocytes can be seen to have dense nuclei with clear pale-blue cytoplasm. Lymphocytes with scanty cytoplasm are small lymphocytes; those with abundant cytoplasm are large lymphocytes. There are normally 1.5-4.0 x 10⁹ lymphocytes per litre of blood. They are involved in immunity and can be subdivided into B-lymphocytes (or B-cells), which produce circulating antibodies, and T-lymphocytes (or T-cells), which are primarily responsible for cell-mediated immunity. T-lymphocytes can differentiate into helper T-cell or cytotoxic T-cell. There is an increase in the number of lymphocytes in the blood and bone marrow in chronic lymphocytic leukaemia.

• lymphocytic adj.

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lym·pho·cyte (limґfo-sīt) [lympho- + -cyte] any of the mononuclear, nonphagocytic leukocytes, found in the blood, lymph, and lymphoid tissues, that are the body's immunologically competent cells and their precursors. They are divided on the basis of ontogeny and function into two classes, B and T lymphocytes, responsible for humoral and cellular immunity, respectively. Most are small lymphocytes, 7–10 μm in diameter, with a round or slightly indented heterochromatic nucleus that almost fills the cell and a thin rim of basophilic cytoplasm that contains few granules. When activated by contact with antigen, small lymphocytes begin macromolecular synthesis, the cytoplasm enlarges until the cells are 10–30 μm in diameter, and the nucleus becomes less completely heterochromatic; they are then referred to as large lymphocytes or lymphoblasts. These cells then proliferate and differentiate into B and T memory cells and into the various effector cell types, B lymphocytes into plasma cells and T cells into helper, cytotoxic, and suppressor cells. Surface markers identifying the lymphocyte types are shown in the accompanying table. See subentries here and under cell. lymphocytic adj

mIg = membrane immunoglobulin; Thy-1, Ly-1, Ly-2, Ly-3 = mouse T-cell surface antigens; CD1, CD3, CD4, CD5, CD8 = human T-cell surface antigens; FcR = Fc receptors; CR = complement receptors; ± indicates present only on a subset of cells or on cells of certain maturities.