Editing Immune system (section)

===Cell mediated immunity===
{{Further|Cell-mediated immunity}}
There are two major subtypes of T cells: the [[cytotoxic T cell|killer T cell]] and the [[T helper cell|helper T cell]]. In addition there are [[regulatory T cells]] which have a role in modulating immune response.{{sfn | Sompayrac | 2019 | pp=7–8}}

==== 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 />

==== Helper T cells ====
[[File:Activation of T and B cells.png|thumb|right|400px|Activation of macrophage or B cell by T helper cell]]
[[T helper cell|Helper T cells]] regulate both the innate and adaptive immune responses and help determine which immune responses the body makes to a particular pathogen.<ref>{{cite journal | vauthors = Abbas AK, Murphy KM, Sher A | title = Functional diversity of helper T lymphocytes | journal = Nature | volume = 383 | issue = 6603 | pages = 787–93 | date = Oct 1996 | pmid = 8893001 | doi = 10.1038/383787a0 | bibcode = 1996Natur.383..787A | s2cid = 4319699 }}</ref><ref>{{cite book | vauthors = McHeyzer-Williams LJ, Malherbe LP, McHeyzer-Williams MG | title = From Innate Immunity to Immunological Memory | chapter = Helper T cell-regulated B cell immunity | volume = 311 | pages = 59–83 | year = 2006 | pmid = 17048705 | doi = 10.1007/3-540-32636-7_3 | isbn = 978-3-540-32635-9 | series = Current Topics in Microbiology and Immunology }}</ref> These cells have no cytotoxic activity and do not kill infected cells or clear pathogens directly. They instead control the immune response by directing other cells to perform these tasks.{{sfn | Sompayrac | 2019 | p=8}}

Helper T cells express T cell receptors that recognize antigen bound to Class II MHC molecules. The MHC:antigen complex is also recognized by the helper cell's [[CD4]] co-receptor, which recruits molecules inside the T cell (such as [[Lck]]) that are responsible for the T cell's activation. Helper T cells have a weaker association with the MHC:antigen complex than observed for killer T cells, meaning many receptors (around 200–300) on the helper T cell must be bound by an MHC:antigen to activate the helper cell, while killer T cells can be activated by engagement of a single MHC:antigen molecule. Helper T cell activation also requires longer duration of engagement with an antigen-presenting cell.<ref>{{cite journal | vauthors = Kovacs B, Maus MV, Riley JL, Derimanov GS, Koretzky GA, June CH, Finkel TH | title = Human CD8+ T cells do not require the polarization of lipid rafts for activation and proliferation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 99 | issue = 23 | pages = 15006–11 | date = Nov 2002 | pmid = 12419850 | pmc = 137535 | doi = 10.1073/pnas.232058599 | bibcode = 2002PNAS...9915006K | doi-access = free }}</ref> The activation of a resting helper T cell causes it to release cytokines that influence the activity of many cell types. Cytokine signals produced by helper T cells enhance the microbicidal function of macrophages and the activity of killer T cells.{{sfn|Alberts|Johnson|Lewis|Raff|2002|loc=Chapter. [https://www.ncbi.nlm.nih.gov/books/NBK26827/ "Helper T Cells and Lymphocyte Activation"]}} In addition, helper T cell activation causes an upregulation of molecules expressed on the T cell's surface, such as CD40 ligand (also called [[CD154]]), which provide extra stimulatory signals typically required to activate antibody-producing B cells.<ref>{{cite journal | vauthors = Grewal IS, Flavell RA | title = CD40 and CD154 in cell-mediated immunity | journal = Annual Review of Immunology | volume = 16 | issue = 1 | pages = 111–35 | year = 1998 | pmid = 9597126 | doi = 10.1146/annurev.immunol.16.1.111 }}</ref>
==== Gamma delta T cells ====
[[Gamma delta T cell]]s (γδ T cells) possess an alternative T-cell receptor (TCR) as opposed to CD4+ and CD8+ (αβ) T cells and share the characteristics of helper T cells, cytotoxic T cells and NK cells. The conditions that produce responses from γδ T cells are not fully understood. Like other 'unconventional' T cell subsets bearing invariant TCRs, such as [[CD1d receptor|CD1d]]-restricted [[natural killer T cell]]s, γδ T cells straddle the border between innate and adaptive immunity.<ref>{{cite journal | vauthors = Girardi M | title = Immunosurveillance and immunoregulation by gammadelta T cells | journal = The Journal of Investigative Dermatology | volume = 126 | issue = 1 | pages = 25–31 | date = Jan 2006 | pmid = 16417214 | doi = 10.1038/sj.jid.5700003 | doi-access = free }}</ref> On one hand, γδ T cells are a component of adaptive immunity as they [[V(D)J recombination|rearrange TCR genes]] to produce receptor diversity and can also develop a memory phenotype. On the other hand, the various subsets are also part of the innate immune system, as restricted TCR or NK receptors may be used as [[pattern recognition receptor]]s. For example, large numbers of human Vγ9/Vδ2 T cells respond within hours to [[non-peptidic antigen|common molecules]] produced by microbes, and highly restricted Vδ1+ T cells in [[epithelium|epithelia]] respond to stressed epithelial cells.<ref name="Holtmeier W, Kabelitz D 2005 151–83" />