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What exactly is reason/process/mechanism for which our thymus gland starts to shrink with age? The thymus gland is the site of production of T lymphocytes which are the best defence against infection and even cancer cells, but as we mature, the tissue shrinks and gets replaced by nothing but fat.

Whereas our bone marrow (although it gets reduced cellularity) stays mostly intact and keeps producing red blood cells. I scoured the web for answers but didnt got satisfactory answer, with mostly dealing with effects rather than cause. One Quora answer draws speculation that perhaps with old age, thymus cells accumulate mutations so its better for body to reduce it, but it doesnt seems satisfactory.

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  • $\begingroup$ @Kshtij Kumar. Intuitively, it must be supposed that 'thymic involution', a process mediated by circulating sex hormones (post-pubertally), occurs because its principal function as the site for maturation of T-lymphocytes has been largely satisfied once processes of their positive and negative selection have been accomplished. Since these T-cells are both long-lived and exposed to the circulation of blood and lymph, these 2 aspects of T-cell maturation are substantially complete in the thymus itself: its role is fulfilled. Incidentally, the original thymocytes are produced in the bone marrow. $\endgroup$
    – jeremiah
    Apr 30 at 16:34
  • $\begingroup$ If you read the 'Wikipedia' article on the 'thymus' carefully, the mechanisms are mentioned by which these long-lived T-cells are programmed in the thymus to recognise the difference between foreign antigens presented to their surface receptors by the MHC of body cells, and autoantigens deriving within the body itself -- i.e. 'positive selection' ('negative selection' of T-cells reacting to autoantigens results in apoptosis of the thymocyte within the thymic medulla). The inference is that once the bulk of this function is achieved in the thymus, refinement of the process occurs peripherally. $\endgroup$
    – jeremiah
    Apr 30 at 16:50

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