NBME 24 Peptide transporter (TAP) (NBME Answers)

Retired NBME 24 Answers

nbme24/Block 2/Question#45 (reveal difficulty score)
A 2-year-old boy is brought to the physician ...
Peptide transporter (TAP) 🔍 / 📺 / 🌳 / 📖
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+17 upvote downvote
submitted by ∗lsmarshall(465)

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MHC class 1 peptide antigen processing > "Antigen peptides loaded onto MHC I in RER after delivery via TAP (transporter associated with antigen processing)" - First Aid 2019.

Bare lymphocyte syndrome type 2 (BLS II; affecting MHC II) is due to mutations in genes that code for transcription factors that normally regulate the expression (gene transcription) of the MHC II genes. Bare lymphocyte syndrome type 1 (BLS I; affecting MHC I), is much more rare, and is associated with TAP deficiencies.

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tyrionwill in the question, it says absence of MHC-I presenting cells. I guess the meaning is lack of MHC-I. IF TAP is missing or dysfunction (bare lym syn type-1), MHC-I should be there, however Ag cannot be loaded to the MHC-I. Can anyone help me to understand more. +1

peridot @tyrionwill From wiki: "The TAP proteins are involved in pumping degraded cytosolic peptides across the endoplasmic reticulum membrane so they can bind HLA class I. Once the peptide:HLA class I complex forms, it is transported to the membrane of the cell. However, a defect in the TAP proteins prevents pumping of peptides into the endoplasmic reticulum so no peptide:HLA class I complexes form, and therefore, no HLA class I is expressed on the membrane. Just like BLS II, the defect isn't in the MHC protein, but rather another accessory protein." +1

j44n i hate this question because MHCI is on all nucleated cells. So this person is literally a bag of RBC's +1

soccerfan23 @j44n Not quite. It's true that MHC I is on all nucleated cells. Because of the TAP mutation, these cells don't express MHC I on their membranes. But these cells still exist. That is what is meant when the vignette says "flow cytometry shows absence of class I MHC-expressing cells. +1

+14 upvote downvote
submitted by ∗sunshinesweetheart(112)

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ADA mutation = autosomal recessive SCID

IL-2-R gamma chain mutation = XLR SCID

IL-2-R alpha chain (CD25) = T-reg dysfunction; assoc with autoimmunity esp. T1DM, MS

Fas ligand (CD95) mutation = no T cell apoptosis in negative selection in thymic medulla = incr circulation of self-reacting lymphocyte = autoimmune lymphoproliferative syndrome

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+2 upvote downvote
submitted by ∗beltorres23(10)

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https://imgur.com/dWuKwft

Important uworld image

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+0 upvote downvote
submitted by ∗yotsubato(1208)

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Cool another question taken from the list of things not in FA

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charcot_bouchard Actually it is in FA. FA 19 Page 100 - Antigen loaded onto MHC1 in RER after delivery via TAP transporter.... Remember FA is that friend who always say I told you so. +32

yotsubato But not in this context +5

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submitted by ∗niboonsh(409)

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more info about Bare Lymphocyte Syndrome https://ghr.nlm.nih.gov/condition/bare-lymphocyte-syndrome-type-i#inheritance

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