Editing Thymus (section)

===Positive selection===
T cells have distinct T cell receptors. These distinct receptors are formed by process of [[V(D)J recombination]] gene rearrangement stimulated by [[RAG1]] and [[RAG2]] genes.<ref name="MDB2019">{{cite book|last1=Hohl|first1=Tobias M.| veditors = Bennett JE, Dolin R, Blaser MJ |title=Mandell, Douglas, and Bennett's principles and practice of infectious diseases|date=2019|publisher=Elsevier|isbn=9780323482554|edition=9th (online)|chapter=6. Cell mediated defence against infection: Thymic selection of CD4+ and CD8+ T Cells}}</ref> This process is error-prone, and some thymocytes fail to make functional T-cell receptors, whereas other thymocytes make T-cell receptors that are autoreactive.<ref name=":0" /> If a functional T cell receptor is formed, the thymocyte will begin to express simultaneously the cell surface proteins [[CD4]] and [[CD8]].<ref name="MDB2019" />

The survival and nature of the T cell then depends on its interaction with surrounding thymic epithelial cells. Here, the T cell receptor interacts with the MHC molecules on the surface of epithelial cells.<ref name="MDB2019" /> A T cell with a receptor that doesn't react, or reacts weakly, will die by [[apoptosis]]. A T cell that does react will survive and proliferate.<ref name="MDB2019" /> A mature T cell expresses only CD4 or CD8, but not both.<ref name="Robbins9thC6" /> This depends on the strength of binding between the TCR and MHC class 1 or class 2.<ref name="MDB2019" /> A T cell receptor that binds mostly to MHC class I tends to produce a mature "cytotoxic" CD8 positive T cell; a T cell receptor that binds mostly to MHC class II tends to produce a CD4 positive T cell.<ref name=":0" />