T-cell Lymphocytes Definition and Function

The type of white blood cell (leukocyte) responsible for cell-mediated immunity. T-cell lymphocytes come to maturity in the thymus during childhood, which is why they are called T-cells. During the maturation process, T-cell lymphocytes “learn” how to recognize self and nonself antigens so they can distinguish between cells that belong to the body and cells that are foreign. Such a safeguard is necessary to keep T-cell lymphocytes from attacking the body’s own cells.

The thymus destroys lymphocytes that do not learn this distinction. After the thymus releases mature T-cell lymphocytes into the blood circulation, they differentiate into several subtypes. These include

  • Cytotoxic T-cell lymphocytes, also called killer T-cells or CD8 cells, which respond to nonself antigens to kill the cells that bear them
  • Helper T-cells, also called CD4 cells, which release cytokines that stimulate b-cell lymphocyte and cytotoxic T-cell lymphocyte activity
  • Memory T-cells, which carry specific antibodies and circulate in the blood for rapid activation should the same antigen reappear
  • Suppressor T-cells, which call off the immune response when the threat to the body ends

The spleen, the lymph nodes, and the mucosaassociated lymphatic tissue (MALT) throughout the body contain millions of T-cell lymphocytes. T-cell lymphocytes also circulate in the blood and the lymph. T-cell lymphocytes may also be the source of disease, such as in hiv/aids (the VIRUS attaches to CD4 helper T-cells) and cutaneous T-cell lymphoma (CTCL), a form of cancer.

For further discussion of T-cell lymphocytes within the context of the structures and functions of the immune system, please see the overview section “The Immune System and Allergies.”


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