UWORLD question about T-cell neg/pos selection

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Anyone understand the question: TCR are found to have very-high affinity to MHC expressed on thymic stromal cells?

The explanation isn't that clear to me, but I thought that the TCR has to bind the MHC in order for it to have positive selection, otherwise it's not gonna recognize MHC with antigen proteins in the real world...Is the "very high affinity" the key to the answer?

 

[username456789]

I think the very high affinity is the problem. I was tricked on this one too, and the explanation did a poor job explaining it. If it's too high affinity, it's no good. It's like Goldilocks when she broke into some bear family's house and stole their porridge (why was it already made and sitting there if they weren't even home??)

 

[match2011]

TCR with no/high avidity for MHC plus self antigen : apoptosis
TCR with low avidity for MHC plus self antigen : defferentiated

 

[bbydoc]

Anyone understand the question: TCR are found to have very-high affinity to MHC expressed on thymic stromal cells?

The explanation isn't that clear to me, but I thought that the TCR has to bind the MHC in order for it to have positive selection, otherwise it's not gonna recognize MHC with antigen proteins in the real world...Is the "very high affinity" the key to the answer?

you are right about the positive selection, you need some affinity in order to be positively selected. but as you know there are two runs of selections and in the negative selection (the one that follows the positive selection) "high affinity" leads to apoptosis, this prevents autoimmune diseases. just imagine the t-cells running through a detector (affinity detector) before leaving the police academy (thymus). you dont want rambos running around in your system and attacking everything that looks a little strange to them.

...thats how I understood it anyway.

 

[IMGUSMLEStep1]

TCR with no/high avidity for MHC plus self antigen : apoptosis
TCR with low avidity for MHC plus self antigen : defferentiated

That pretty much sums it all up.

 

[mofonyx]

It's a two step process though, but you guys probably already know that. So strong first, then weak affinity.