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lymphocyte (lym·pho·cyte) (lim´fo-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.
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amplifier T lymphocyte a T-lymphocyte that modifies a developing immune response by releasing nonspecific signals to which other T-lymphocytes (either effector or suppressor cells) respond.
B lymphocytes bursa-dependent lymphocytes in birds and their counterparts in nonavian vertebrates including human beings, the cells primarily responsible for humoral immunity, the precursors of antibody-producing cells ( plasma cells). In birds maturation of these cells takes place in the bursa of Fabricius; the hypothesized analogous tissue in other vertebrates was termed the “bursa-equivalent” tissue. It now appears that B lymphocyte maturation occurs primarily in the bone marrow in mammals. B lymphocytes are characterized by the presence of surface immunoglobulin, monomeric IgM or IgD, which constitutes the B-cell antigen receptors. When stimulated by antigen, a process that requires the cooperation of helper T cells and macrophages, B lymphocytes proliferate and differentiate into plasma cells and memory B cells. The entire clone of cells descended from a single activated B lymphocyte produces immunoglobulins having the same antigen combining site as that in the antigen receptors of the original cell; thus all of the antibody produced and all of the memory cells are specific for the antigen that induced their formation. Called also B cells.
CD4 T lymphocytes , CD4+ T lymphocytes CD4 cells.
CD8 T lymphocytes , CD8+ T lymphocytes CD8 cells.
cytotoxic T lymphocytes(CTL) differentiated T lymphocytes that can recognize and lyse target cells bearing specific antigens recognized by their antigen receptors. Recognition is MHC restricted; the foreign antigen is recognized only in association with self MHC antigens. The cytotoxic activity requires firm binding of the lymphocyte to the target cell to produce holes in the plasma membrane of the target cell, loss of its cell content, and osmotic lysis. These lymphocytes are important in graft rejection and killing of tumor cells and virus-infected host cells. Murine killer T cells are marked by the Ly-2 and Ly-3 antigens, human cells by the CD4 and CD8 antigens. Called also killer or killer T cells.
helper T lymphocytes helper T cells.
large lymphocyte see lymphocyte.
large granular lymphocytes lymphocytes marked by the presence of large granules visible by light microscopy, responsible for most natural killer cell activity.
plasmacytoid lymphocyte a cell morphologically resembling a small lymphocyte but having a well-developed rough endoplasmic reticulum like that of a plasma cell; frequently observed in the blood of patients with plasma cell dyscrasias or hypergammaglobulinemia.
Rieder lymphocyte Rieder cell.
small lymphocyte see lymphocyte.
T lymphocytes the cells primarily responsible for cell-mediated immunity; they originate from lymphoid stem cells that migrate from the bone marrow to the thymus and differentiate under the influence of the thymic hormones thymopoietin and thymosin. They are characterized by specific surface antigens: the pan-T antigens Thy-1 (murine) and CD3 (human) are found on all mature T lymphocytes; other markers characterize T cell subsets. T cell antigen receptors are triggered by antigen only when associated with self MHC antigens, e.g., by antigens processed and presented by macrophages, viral antigens on the surface of host cells, and tumor neoantigens. When activated by antigen, T lymphocytes proliferate and differentiate into T memory cells and the various types of regulatory and effector T cells; see cytotoxic T l's and helper, suppressor, contrasuppressor, and TDTH cells, under cell. See accompanying table. Called also T cells and thymus-dependent l's.
thymus-dependent lymphocytes T l's.
thymus-independent lymphocytes B l's.
tumor-infiltrating lymphocytes(TIL) lymphocytes isolated from the inflammatory infiltrate present in solid tumors and cultured in interleukin-2 (IL-2); they have specific activity against the tumor from which they are derived.
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