Helper T Cell
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Helper T cells, also called T-hepler cells, are T lymphocytes that provide help to other effector cells in the immune response by recognizing foreign antigens and secreting cytokines that activate T and B cells. Helper T cells belong to the CD4+ T cell subset that express the surface protein CD4. Helper T cells have no cytotoxic or phagocytic activity; they cannot kill infected host cells or pathogens. They are involved in activating and directing other immune cells, and are particularly important in the immune system. Helper T cells are essential in determining B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages.
Four kinds of helper T cells have been identified, Th1, Th2, Th17 and Tfh (follicular B helper T, or just follicular helper T) cells. Th1 cells are targeted towards intracellular pathogens such as bacteria and parasites via the activation of infected macrophages, while Th2 cells invoke antibody production in B-cells, which neutralise extracellular pathogens and toxins. Th2 cells are essential for the production of IgE antibodies and perhaps assist in the production of other classes as well. Th17 cells protect surfaces against extracellular bacteria. Th17 cells are a newly discovered subset of helper T cells producing interleukin 17. They are considered developmentally distinct from Th1 and Th2 cells and excessive amounts of the cell are thought to play a key role in autoimmune disease. Tfh cells are found in B cell follicles and mediate antigen specific naïve or memory B cell activation, which triggers germinal center formation, probably through the expression of CD40L and the secretion of IL-21 and IL-4.