lymphocyte

A mononuclear, nongranular leukocyte having a deeply staining nucleus containing dense chromatin and a sparse, pale-blue-staining cytoplasm. It participates in immunity.

• activated l. — one that has reacted on exposure to antigen or to a mitogen.
• l.-activating antigen — see lymphocyte-defined antigen (below).
• B l's — thymus-independent lymphocytes, which develop from stem cells in hematopoietic tissue, including the blood islands of the fetal yolk sac, the fetal liver and spleen, and the bone marrow. The B in B lymphocyte refers to the bursa of Fabricius, an organ in birds where B cell differentiation occurs. No analogous organ has been found in mammals, though an equivalent is assumed to exist—probably bone marrow.
— B lymphocytes are involved in humoral immunity, i.e. the production of antibodies. A small, mature B lymphocyte can be activated by the binding of an antigen to specific cell-surface receptors. This induces proliferation of the cell, resulting in an expanded clone of cells specific for that antigen. These cells can then differentiate and begin to secrete immunoglobulin (Ig) molecules. Antibody production for most antigens involves interaction with helper T lymphocytes. All of the cells of a single clone secrete Ig that binds specifically to the antigen that stimulated the production of antibody. Mature antibody-secreting cells are plasma cells. Following blastogenesis some of the cells produced by division revert to small lymphocytes and remain as a longlived clone of ‘memory’ cells.
• l. blastogenesis — increased DNA synthesis followed by cell division and differentiation of lymphocytes in response to antigens or mitogens; an in vitro test of lymphocyte function.
• l.-defined antigen — histocompatibility antigens found on lymphocytes, macrophages, epidermal cells and sperm, capable of stimulating division in allogeneic lymphocytes. Called also lymphocyte-activating antigen.
• helper l's — a subset of T lymphocytes that is itself comprised of Th1 and Th2 cells that cooperate with and regulate B lymphocytes in the antibody response and with themselves and other effector T lymphocytes.
• histiocytic l. — see prolymphocyte.
• intraepidermal l's — cellular components of the cutaneous immune system.
• intraepithelial l's — cellular components of the mucosal immune system.
• large l. — lymphoblast.
• large granular l. — see natural killer cell.
• memory l. — see memory cell.
• natural killer (NK) l. — see natural killer cell.
• non-T, non-B l. — see null cell.
• pre-B l. — a progenitor B cell derived from bone marrow stem cells.
• resting l. — small lymphocytes in the G₀ stage of the cell cycle, prior to activation.
• small l. — unstimulated B and T lymphocytes.
• l. stimulation test — see lymphocyte blastogenesis (above).
• suppressor (Ts) l. — see T lymphocytes (below).
• T l's — lymphocytes produced from pluripotential bone marrow stem cells which migrate during late embryogenesis to the thymus. Enormous numbers of cells are produced in the thymus, most of which are destroyed there. Less than 10% of cells which are non-self reactive leave the thymus to populate the T lymphocyte areas of the secondary lymphoid tissues (lymph nodes, spleen, Peyer's patches, etc.). At least four subsets of T lymphocytes are defined by cell surface markers and more especially by function.
— Three of the subsets are regulatory cells called T helper (Th) 1 and 2, and T suppressor (Ts) lymphocytes. The other subset is an effector cell called cytotoxic T (Tc) lymphocytes or CTLs. Following antigen binding to antigen-specific T cell receptor (TCR) molecules, T lymphocytes, like B lymphocytes, undergo blastogenesis, in which they enlarge and divide. Some daughter cells revert to form an expanded clone of memory T lymphocytes. For both regulatory and effector functions, T lymphocytes produce a number of substances generally referred to as lymphokines. Lymphokines, unlike antibody, do not bind specifically with antigen, but rather they direct cell functions.
• thymus-derived l. — see T lymphocytes (above).
• l. transfer test — injection of lymphocytes from a potential graft recipient into the skin of a potential donor can be a test of histocompatibility, primarily in humans at the LD loci. Noncompatibility is marked by an inflammatory skin reaction.
• l. transformation test — see lymphocyte blastogenesis (above).
• l. transforming factor (LTF) — an obsolete term for a lymphokine that causes transformation and clonal expansion of nonsensitized lymphocytes.
• tumor-infiltrating l's (TILs) — T lymphocyte preparations from surgically removed cancer tissue that may be activated in vitro and returned to the host as a basis for specific anticancer therapy.