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Retired NBME 20 Answers

nbme20/Block 3/Question#46 (reveal difficulty score)
Removal of the thymus at birth results in ...
Thymic lymphocytes produced before thymectomy are long-lived ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
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submitted by โˆ—hayayah(1212)
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By age 75, the thymus is little more than fatty tissue. Fortunately, the thymus produces all of your T cells by the time you reach puberty. They are long-lived and that's why you can lose your thymus without impairment of your immune system.

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sweetmed  Memory T cells live for six months or less in healthy humans (Westera et al., 2013), whereas naive T cells can live for up to nine years +8
whossayin  so the bone marrow does not take the role of the thymus? +3
dr_jan_itor  @sweetmed, does that mean that if someone loses their thymus, they would develop imunodeficiencies appx 9 years later as the naive T cells have died off? +10
hpsbwz  @dr_jan_itor no, because once all of the thymocytes become T-lymphocytes, they are stored in lymphoid organs until they're needed. this is why removal of the thymus in MG does not cause any immune system deficiency. +9
peridot  @dr_jan_itor From wiki: "Thymic involution results in a decreased output of naรฏve T lymphocytes โ€“ mature T cells that are tolerant to self antigens, responsive to foreign antigens, but have not yet been stimulated by a foreign substance. In adults, naรฏve T-cells are hypothesized to be primarily maintained through homeostatic proliferation, or cell division of existing naรฏve T cells. Though homeostatic proliferation helps sustain TCR even with minimal to nearly absent thymic activity, it does not increase the receptor diversity." +6



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submitted by โˆ—lulumomovicky(4)
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Thymic output of T cell repertoire during the growing phases is vital, but it becomes unnecessary for repertoire maintenance during adulthood. This happens because the T cell regeneration in adulthood is almost entirely derived from homeostatic proliferation of the EXISTING T cell pool, which is sufficient to maintain a large compartment of naive CD4 T cells. Thymic T cell generation can add new naive T cells and enrich diversity, while homeostatic T cell proliferation can sustain the richness of the TCR repertoire already created. This means that the Thymic lymphocytes produced before thymectomy are long-lived naive T cells, which are maintained stable in quantity thanks to Homeostatic proliferation in adulthood.

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