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). After leaving the thymus the mature T cells continually circulate between lymph nodes and the bloodstream as naive T cells. Eventually a naive T cell is presented with antigen by an antigen-presenting cell (e.g. dendritic cell or macrophage) within lymphoid tissue and becomes ‘armed’ to act rapidly as an effector T cell when it subsequently encounters its specific antigen. Subpopulations of T cells play different roles in the immune response and can be characterized by their surface antigens (see cd). Helper T cells carry the CD4 antigen on their surface, and recognize foreign antigens provided these are presented by cells (such as macrophages and B cells) bearing MHC class II proteins. The helper T cell binds to its target cell by means of T-cell receptors. Helper T cells are essential in the majority of infections for stimulating B cells to proliferate and differentiate into clones of antibody-producing plasma cells. They also ensure that bacteria ingested by macrophages do not survive and grow inside the host cells.

Cytotoxic T cells, which carry the CD8 antigen, recognize foreign antigen bound with MHC class I protein on the surface of virus-infected cells and destroy the cell by releasing cytolytic proteins. Regulatory T cells (T_reg or suppressor T cells) are important in regulating the activity of other lymphocytes and are crucial in maintaining tolerance to self tissues. See also immunological memory.