Editing T cell (section)

===Thymic output===
About 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immunocompetent T cells.<ref>{{Cite book|last=Murphy|first=Kenneth|title=Janeway's Immunobiology|publisher=Garland Science|year=2011|isbn=9780815342434|edition=8th|pages=297}} </ref>
The thymus contributes fewer cells as a person ages. As the thymus shrinks by about 3%<ref name="pmid10837068">{{cite journal|vauthors=Haynes BF, Markert ML, Sempowski GD, Patel DD, Hale LP|title=The role of the thymus in immune reconstitution in aging, bone marrow transplantation, and HIV-1 infection|journal=Annu. Rev. Immunol.|volume=18|pages=529–560|year=2000|pmid=10837068|doi=10.1146/annurev.immunol.18.1.529}}</ref> a year throughout middle age, a corresponding fall in the thymic production of naive T cells occurs, leaving peripheral T cell expansion and regeneration to play a greater role in protecting older people.