Editing T cell (section)

====Positive selection====
The process of positive selection takes 3 to 4 days and occurs in the thymic cortex.<ref>{{cite journal|vauthors = Ross JO, Melichar HJ, Au-Yeung BB, Herzmark P, Weiss A, Robey EA|title = Distinct phases in the positive selection of CD8+ T cells distinguished by intrathymic migration and T-cell receptor signaling patterns|journal = Proceedings of the National Academy of Sciences of the United States of America|volume = 111|issue = 25|pages = E2550–E2558|date = June 2014|pmid = 24927565|pmc = 4078834|doi = 10.1073/pnas.1408482111|doi-access = free|bibcode = 2014PNAS..111E2550R}}</ref> Double-positive thymocytes (CD4<sup>+</sup>/CD8<sup>+</sup>) migrate deep into the [[Thymus#Structure|thymic cortex]], where they are presented with self-[[antigen]]s. These self-antigens are expressed by [[Cortical thymic epithelial cells|thymic cortical epithelial cells]] on MHC molecules, which reside on the surface of cortical epithelial cells. Only thymocytes that interact well with MHC-I or MHC-II will receive a vital "survival signal", while those that cannot interact strongly enough will receive no signal and [[Apoptosis|die from neglect]]. This process ensures that the surviving thymocytes will have an 'MHC affinity' that means they will exhibit stronger binding affinity for specific MHC alleles in that organism.<ref>{{Cite journal |last1=Štefanov́a |first1=Irena |last2=Dorfman |first2=Jeffrey R. |last3=Tsukamoto |first3=Makoto |last4=Germain |first4=Ronald N. |date=February 2003 |title=On the role of self-recognition in T cell responses to foreign antigen |url=https://onlinelibrary.wiley.com/doi/10.1034/j.1600-065X.2003.00006.x |journal=Immunological Reviews |language=en |volume=191 |issue=1 |pages=97–106 |doi=10.1034/j.1600-065X.2003.00006.x |pmid=12614354 |issn=0105-2896|url-access=subscription }}</ref> The vast majority of developing thymocytes will not pass positive selection, and die during this process.<ref>{{cite journal|vauthors = Starr TK, Jameson SC, Hogquist KA|title = Positive and negative selection of T cells|journal = Annual Review of Immunology|volume = 21|issue = 1|pages = 139–176|date = 2003-01-01|pmid = 12414722|doi = 10.1146/annurev.immunol.21.120601.141107}}</ref>

A thymocyte's fate is determined during positive selection. Double-positive cells (CD4<sup>+</sup>/CD8<sup>+</sup>) that interact well with MHC ''class II'' molecules will eventually become CD4<sup>+</sup> "helper" cells, whereas thymocytes that interact well with MHC ''class I'' molecules mature into CD8<sup>+</sup> "killer" cells. A thymocyte becomes a CD4<sup>+</sup> cell by down-regulating expression of its CD8 cell surface receptors. If the cell does not lose its signal, it will continue downregulating CD8 and become a CD4<sup>+</sup>, both CD8<sup>+</sup> and CD4<sup>+</sup> cells are now ''single positive'' cells.<ref>{{cite journal|vauthors=Zerrahn J, Held W, Raulet DH|title=The MHC reactivity of the T cell repertoire prior to positive and negative selection|journal=Cell|volume=88|issue=5|pages=627–636|date=March 1997|pmid=9054502|doi=10.1016/S0092-8674(00)81905-4|s2cid=15983629|doi-access=free}}</ref>

This process does not filter for thymocytes that may cause [[autoimmunity]]. The potentially autoimmune cells are removed by the following process of negative selection, which occurs in the thymic medulla.