Cell‐mediated immunity depends upon the activity of T‐lymphocytes. T‐lymphocytes have a longer life span than B‐lymphocytes and are found in the same lymphatic tissues as the B‐lymphocytes. The T‐lymphocytes react with certain antigenic determinants and become immunologically “committed.” Part of this commitment is the conversion to a subset of cells called cytotoxic T‐lymphocytes.
Activity of cytotoxic T-lymphocytes. Cytotoxic T-lymphocytes do not produce antibody molecules. Rather, they leave the lymphatic tissues and enter the circulation. They circulate through the blood vessels and gather at the infection site. Here they interact directly with organisms such as fungi, protozoa, cancer cells, and transplant cells. They also interact with virus-infected cells and bacteria-infected cells (such as lung cells infected with tuberculosis). The T-lymphocytes exert a “lethal hit” on the cells and secrete substances into them that lead to cellular destruction.
In addition to their direct interaction, T-lymphocytes also secrete substances calledlymphokines. Lymphokines attract phagocytes to the area and encourage them to perform phagocytosis on fungi, protozoa, and infected cells. This activity helps relieve the infection. Lymphokines are also known as cytokines. An important cytokine isinterleukin-1, which activates T-lymphocytes, causing them to proliferate further and form clones.
Helper and suppressor T-lymphocytes. Helper T-lymphocytes also function in the immune system by encouraging the activity of B-lymphocytes in the production of antibodies. Suppressor T-lymphocytes regulate or suppress the activity of the immune system so that it is not excessive. Natural killer (NK) cells are T-lymphocytes that recognize and destroy many types of target cells without being exposed to antigens. Technically, these are not part of the specific immune response. Finally, the delayed hypersensitivity T-lymphocytes function in hypersensitivity reactions and encourage local tissue inflammations.