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NBME 20 Answers

nbme20/Block 3/Question#46 (40.0 difficulty score)
Removal of the thymus at birth results in ...
Thymic lymphocytes produced before thymectomy are long-lived🔍

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submitted by hayayah(1056),
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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 +6  
whossayin  so the bone marrow does not take the role of the thymus? +1  
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? +8  
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. +6  
peridot  @dr_jan_itor From wiki: "Thymic involution results in a decreased output of naïve T lymphocytes – 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, naïve T-cells are hypothesized to be primarily maintained through homeostatic proliferation, or cell division of existing naïve T cells. Though homeostatic proliferation helps sustain TCR even with minimal to nearly absent thymic activity, it does not increase the receptor diversity." +3