T cell (T lymphocyte) Any of a population of lymphocytes that are the principal agents of cell-mediated immunity. T cells are derived from the bone marrow but migrate to the thymus to mature (hence T cell). Subpopulations of T cells play different roles in the immune response and can be characterized by their surface antigens (see CD). Helper T cells recognize foreign antigens provided these are presented by cells (such as macrophages and B lymphocytes) bearing class II histocompatibility (MHC) proteins (see histocompatibility). The helper T cell carries T-cell receptors, which recognize the class II MHC proteins on the antigen-presenting cell. Interleukin-1 released by inducer cells stimulates helper T cells to release interleukin-2, which in turn triggers the release of other cytokines (see interleukin). Consequently there is a proliferation of B lymphocytes and the generation of effector T cells, i.e. cytotoxic T cells, delayed hypersensitivity T cells, and suppressor T cells.

Cytotoxic T cells recognize foreign antigen on the surface of virus-infected cells and destroy the cell by releasing cytolytic proteins. Suppressor T cells are important in regulating the activity of other lymphocytes and are crucial in maintaining tolerance to self tissues. Delayed hypersensitivity T cells mediate delayed hypersensitivity by releasing various lymphokines.

T cell A type of lymphocyte manufactured in the thymus gland (hence the name) that is involved in the cell-mediated response. There are several classes of T cell including: helper T cells, which aid B cells in antibody secretion; suppressor T cells, which reduce the immunological responses of B and T cells; cytotoxic T cells, which attack virally infected or cancerous cells; and delayed hypersensitivity T cells, which participate in the delayed hypersensitivity response.

T cell (also T-cell) • n. Physiol. a lymphocyte of a type produced or processed by the thymus gland and actively participating in the immune response. Also called T lymphocyte. Compare with B cell.