This T cell (blue), one of the immune system's principal means of defense, identifies the molecular signature of a dendritic cell (green) at a contact point called the immunological synapse. If the immunological synapse signals the presence of a foe, the T cell will attack. Credit: Lawrence Berkeley Lab.
A T-lymphocyte, also called a T-cell, is a type of lymphocyte (which, in turn, is a type of white blood cell) which plays an important role in the immune system. T-lymphocytes are formed in the thymus gland and react highly specifically against the particular type of antigen that initiated their development. T-lymphocytes account for more than four-fifths of circulating lymphocytes.
There are three main groups of T-lymphocytes: killer (cytotoxic) cells, helper cells, and suppressor cells. The killer T-lymphocytes (like B-lymphocytes) are sensitized and stimulated to multiply by the presence of antigens, in this case by antigens present on abnormal body cells (e.g. cells that have been invaded by viruses, cells in transplanted tissue, and tumor cells). Unlike the B-lymphocytes, the killer cells do not produce antibodies; instead, they travel to and attach to the cells recognized as abnormal. The killer cells then release chemicals known as lympokines, which help destroy the abnormal cells. This is called cell-mediated immunity.
Helper T-cells enhance the activities of the killer T-cells and B-cells, and also control other aspects of the immune response. In people who are infected with HIV (the AIDS virus) these helper T-cells are reduced in number, thus impairing the body's ability to fight certain types of infections and tumors. Suppressor T-cells have the effect of switching off the immune response.