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NBME 20 Answers

nbme20/Block 2/Question#41

Purified serum antibodies elicited by immunization ...

Proteins X and Y express the same epitopes

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submitted by usmile1(21),

If you look at Uworld question ID 12299 it has a wonderful explanation for this. If they share the same epitopes, it will have a downward slope. If they share none of the same epitopes, the line will be horizontal across the graph (indicating no change as the amount of Y added increases)

eacv  omg YES!! thanks Uworld I got it correct! exactly this qx asked the exact opposite thing! Hahaha I loved it !! +3  

 +1  upvote downvote
submitted by amarousis(10),

so how would the graph look for protein x expresses all of the epitopes expressed by protein y, but protein y does not express all of the epitopes expressed by protein x?

drw  then some anti-X cannot relocate to Y even Y is added at whatever high dose. at this condition, the line can never touch the axis-Y. on the contrary, if Y express all epitopes on the X, but X does not express all epitopes on the Y, that means some Y epitopes are not seen on X. at this condition, I don't know what will be the line looked like. +1  

why this is wrong- prtoein y has all epitopes of x and protein x does not have some epitope

+/- prudvi96(0),

The graph actually did not touch axis-y when dose of Y is low And at some point, the bounded X started to drop How to explain the bounded X did not drop before that Y dose level is reached?

+/- ssc505684708(0),

I found this, I think it gives kinda of an explanation: If the regression of log ((A'/A) -1) on -log B is linear with a slope of -1, then this indicates that the antagonism is competitive and by definition the agonist and antagonist act at the SAME recognition sites. So basically the more you add Y, the less X is bound, which means they have a same structural component (epitope) and must act on the same site? Don't know if this makes sense...


High X is bound when low Y is added, and low X is bound when Y is added. So MAYBE they are competing for the same binding spot/epitope due to this relationship (epitope= antibody binding site) ??????????

 +0  upvote downvote
submitted by breis(14),

your guess is as good as mine.....................................................................

nala_ula  I spent so long on this question and same... hahaha +  

Ran across a question similar to this in another question bank. In that question, there was no change as the concentration of the new protein was added, which meant that the two proteins did not share similar epitopes. In this question, the antibody is just trying to bind its epitope, so adding more or y means less binding of protein x because the antibody favors the proteins the same.