Editing Immune system (section)

==== 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>