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The development of t-lymphocytes are done by thymus and we also know that the thymus degenerates before puberty and we also know the maximum lifespan of WBC's is 15 days. So, how do t-lymphocytes become mature if thymus has lost his function?

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A few points, I'll expand on this later and add references if I have time:

  1. The thymus starts degenerating at puberty but continues to function in thymocyte maturation for decades, albeit at a lower level
  2. Many mature T cells can live for years
  3. Through homeostatic proliferation, peripheral T cells maintain their population size
  4. Immune surveillance generally decreases with age

Note that extended lifespans and homeostatic proliferation (points 2 and 3) don't contribute to TCR diversity but only maintain the existing repertoire.

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The key thing is this: yes, the thymus involutes at puberty but that doesn't halt thymopoiesis. The thymus continues to produce naive T lymphocytes throughout our lives (even up to 70 years of age-see references) albeit at a very reduced rate. This mechanism is very important for example in HIV-infected individuals who are still able to repopulate their immune cells through a combination of peripheral T cell division and thymic production of naive T cells.

References: Reference_1, Reference_2, Reference_3

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