T-cell: One type of white blood cell that attacks virus-infected cells, foreign cells, and cancer cells. T cells also produce a number of substances that regulate the immune response. Also called T lymphocyte.
Source: MedLinePlus Magazine (NIH)1
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T cells have a variety of roles and are classified by subsets. T cells are divided into two broad categories: CD8+ T cells or CD4+ T cells, based on which protein is present on the cell's surface. T cells carry out multiple functions, including killing infected cells and activating or recruiting other immune cells.
CD8+ T cells also are called cytotoxic T cells or cytotoxic lymphocytes (CTLs). They are crucial for recognizing and removing virus-infected cells and cancer cells. CTLs have specialized compartments, or granules, containing cytotoxins that cause apoptosis, i.e., programmed cell death. Because of its potency, the release of granules is tightly regulated by the immune system.
The four major CD4+ T-cell subsets are TH1, TH2, TH17, and Treg, with "TH" referring to "T helper cell." TH1 cells are critical for coordinating immune responses against intracellular microbes, especially bacteria. They produce and secrete molecules that alert and activate other immune cells, like bacteria-ingesting macrophages. TH2 cells are important for coordinating immune responses against extracellular pathogens, like helminths (parasitic worms), by alerting B cells, granulocytes, and mast cells. TH17 cells are named for their ability to produce interleukin 17 (IL-17), a signaling molecule that activates immune and non-immune cells. TH17 cells are important for recruiting neutrophils.
Regulatory T cells (Tregs), as the name suggests, monitor and inhibit the activity of other T cells. They prevent adverse immune activation and maintain tolerance, or the prevention of immune responses against the body's own cells and antigens.
Source: NIAID (NIH)2
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- Source: MedLinePlus Magazine (NIH): medlineplus.gov/ magazine/ issues/ summer08/ articles/ summer08pg7-8.html
- Source: NIAID (NIH): niaid.nih.gov/ topics/ immuneSystem/ Pages/ immuneCells.aspx
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