Editing Immune system (section)

==== Killer T cells ====
[[Killer T cells]] are a sub-group of T cells that kill cells that are infected with viruses (and other pathogens), or are otherwise damaged or dysfunctional.<ref>{{cite journal | vauthors = Harty JT, Tvinnereim AR, White DW | title = CD8+ T cell effector mechanisms in resistance to infection | journal = Annual Review of Immunology | volume = 18 | issue = 1 | pages = 275–308 | year = 2000 | pmid = 10837060 | doi = 10.1146/annurev.immunol.18.1.275 }}</ref> As with B cells, each type of T cell recognizes a different antigen. Killer T cells are activated when their [[T-cell receptor]] binds to this specific antigen in a complex with the MHC Class I receptor of another cell. Recognition of this MHC:antigen complex is aided by a [[co-receptor]] on the T cell, called [[CD8]]. The T cell then travels throughout the body in search of cells where the MHC I receptors bear this antigen. When an activated T cell contacts such cells, it releases [[cytotoxicity|cytotoxins]], such as [[perforin]], which form pores in the target cell's [[cell membrane|plasma membrane]], allowing [[ion]]s, water and toxins to enter. The entry of another toxin called [[granulysin]] (a protease) induces the target cell to undergo [[apoptosis]].<ref name=Radoja>{{cite journal | vauthors = Radoja S, Frey AB, Vukmanovic S | title = T-cell receptor signaling events triggering granule exocytosis | journal = [[Critical Reviews in Immunology]] | volume = 26 | issue = 3 | pages = 265–90 | year = 2006 | pmid = 16928189 | doi = 10.1615/CritRevImmunol.v26.i3.40 }}</ref> T cell killing of host cells is particularly important in preventing the replication of viruses. T cell activation is tightly controlled and generally requires a very strong MHC/antigen activation signal, or additional activation signals provided by "helper" T cells (see below).<ref name=Radoja />