Editing T cell (section)

{{Short description|White blood cells of the immune system}}
{{Infobox cell
|Name = T cell
|Latin = lymphocytus T
|Image = Blausen 0625 Lymphocyte T cell (crop).png
|Caption = 3D illustration of a T cell
|Image2 = Red White Blood cells.jpg
|Caption2 = Scanning electron micrograph of a [[red blood cell]] (left), a [[platelet]] (center), and a T lymphocyte (right); colorized
|Precursor = 
|System = [[Immune system]]
}}

'''T cells''' are one of the important types of [[white blood cell]]s of the [[immune system]] and play a central role in the [[adaptive immune response]]. T cells can be distinguished from other [[lymphocytes]] by the presence of a [[T-cell receptor]] (TCR) on their [[cell surface receptor|cell surface]].

T cells are born from [[hematopoietic stem cells]],<ref>{{Cite web|title=5. Hematopoietic Stem Cells|url=https://stemcells.nih.gov/info/2001report/chapter5.htm|archive-url=https://web.archive.org/web/20161029020128/https://stemcells.nih.gov/info/2001report/chapter5.htm|archive-date=29 October 2016|url-status=dead|work=Stem Cell Information|location=Bethesda, MD|publisher=National Institutes of Health, U.S. Department of Health and Human Services|date=17 June 2001|access-date=21 December 2021}}</ref> found in the [[bone marrow]]. Developing T cells then migrate to the [[thymus]] gland to develop (or mature). T cells derive their name from the [[thymus]].<ref name = "Alberts_2002"/><ref>{{Cite book|vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P|date=2002|chapter = Helper t Cells and Lymphocyte Activation|page = 1367|chapter-url=https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4&part=A4422|title = Molecular Biology of the Cell|publisher=Garland Science|edition=4th|language=en|quote = T cells ... derive their [name] from the organs in which they develop. T cells develop [mature] in the thymus}}<!--N.B.: Contrary to this reference, B cells derive their name not from (human) bone marrow, but from the bursa (found in birds): {{cite journal|vauthors = Cooper MD|title = The early history of B cells|journal = Nature Reviews. Immunology|volume = 15|issue = 3|pages = 191–197|date = March 2015|pmid = 25656707|doi = 10.1038/nri3801|author-link = Max Dale Cooper|doi-access = free}}--></ref> After migration to the thymus, the precursor cells mature into several distinct types of T cells. T cell differentiation also continues after they have left the thymus. Groups of specific, differentiated T cell subtypes have a variety of important functions in controlling and shaping the [[immune response]].

One of these functions is immune-mediated cell death, and it is carried out by two major subtypes: [[Cytotoxic T cell|CD8<sup>+</sup> "killer"]] (cytotoxic) and [[T helper cell|CD4<sup>+</sup> "helper"]] T cells. (These are named for the presence of the cell surface proteins [[CD8]] or [[CD4]].) CD8<sup>+</sup> T cells, also known as "killer T cells", are [[Cytotoxicity|cytotoxic]] – this means that they are able to directly kill virus-infected cells, as well as cancer cells. CD8<sup>+</sup> T cells are also able to use small signalling proteins, known as [[cytokine]]s, to recruit other types of cells when mounting an immune response. A different population of T cells, the CD4<sup>+</sup> T cells, function as "helper cells". Unlike CD8<sup>+</sup> killer T cells, the CD4<sup>+</sup> helper T (T<sub>H</sub>) cells function by further activating [[memory B cell]]s and cytotoxic T cells, which leads to a larger immune response. The specific adaptive immune response regulated by the T<sub>H</sub> cell depends on its subtype (such as T-helper1, T-helper2, T-helper17, regulatory T-cell),<ref>{{cite journal|vauthors = Luckheeram RV, Zhou R, Verma AD, Xia B|title = CD4⁺T cells: differentiation and functions|journal = Clinical & Developmental Immunology|volume = 2012|pages = 925135|date = 2012|pmid = 22474485|pmc = 3312336|doi = 10.1155/2012/925135|doi-access = free}}</ref> which is distinguished by the types of cytokines they secrete.<ref name = "Alberts_2002">{{Cite book|vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P|date=2002|chapter = Helper T Cells and Lymphocyte Activation|chapter-url= https://www.ncbi.nlm.nih.gov/books/NBK26827/|title = Molecular Biology of the Cell|publisher=Garland Science|edition=4th|language=en}}</ref>

[[Regulatory T cell]]s are yet another distinct population of T cells that provide the critical mechanism of [[Immune tolerance|tolerance]], whereby [[immune cells]] are able to distinguish invading cells from "self". This prevents immune cells from inappropriately reacting against one's own cells, known as an "[[Autoimmunity|autoimmune]]" response. For this reason, these regulatory T cells have also been called "suppressor" T cells. These same regulatory T cells can also be co-opted by cancer cells to prevent the recognition of, and an immune response against, tumor cells.