Bio Systems Questions EK Pg141

[icydragon123]

---

Members do not see this ad.

.

 

[Alpha-ketogluterate]

Hello SDN!!! I have a question Im really confused about from the EK Bio systems book. Pls help! Thanks!

Negative selection takes place in the corticomedullary junction of the thymus where developing T cells interact with medullary thymic epithelial cells (mTECs). The immature T cells would be expected to display:

A) Self-antigen on class I MHCs
B) Self-antigen on Class II MHCs
C) non-self antigen on Class II MHCs
D) Self-antigen on Class I and Class II MHCs

Answer is D.


My problem with this question is that I dont understand why T cells would display self antigens. I thought that the T cells would recognize self antigens on Class I and Class II MHCs and if they interact then they undergo apoptosis. Why would T cells display these self antigens?

Sorry to revive this question from last year but I have exactly the same question. I do not think this question is worded right.
Let's examine the options:

A) Okay this is plausible, although negative clonal selection is about making sure the cells are not over reacting to self antigens, not the display of the self antigen BY the T cell itself.
B) MHC 2 do NOT display self antigens, so this is FALSE.
C) T cells do NOT have MHC 2, only macrophages, denderitic cells and some B cells have MHC 2, so this is FALSE.
D) Again Self antigens are NOT displayed on MHC 2, so this too is FALSE, in addition, just as with option A, I do not see how the presentation of self antigen is itself relevant to negative selection. It is the reaction to the self antigen which is important.

I think A is the best answer even though it also does not quite make sense. D is the book's proposed answer and the 2nd part of it by definition is incorrect.

Perhaps someone has more insight into this?

 

[PhilzCoffeeAddict]

I think the question is meh in terms of trying to get the concept that T cells either possess MHC1 or MHC2. Knowing that T cells display either should have hinted that D is the correct answer.

In terms of negative selection. I think the point they were trying to make is that if cells displayed either of the self-antigen it would NOT select (even though it recognizes the self-antigen) it out. I think its a meh question in terms of wording because this could be misinterpreted to be positive selection.

 

[Alpha-ketogluterate]

I think the question is meh in terms of trying to get the concept that T cells either possess MHC1 or MHC2. Knowing that T cells display either should have hinted that D is the correct answer.

In terms of negative selection. I think the point they were trying to make is that if cells displayed either of the self-antigen it would NOT select (even though it recognizes the self-antigen) it out. I think its a meh question in terms of wording because this could be misinterpreted to be positive selection.

I agree with you that the question is miss-worded but: EK (as well as Kaplan and a quick google search) explicitly state that only Denderitic cells, macrophages and some B cells use MHC2 (as in are able to express it, we are not talking about it's recognition), T cells DO NOT. My biochem teacher said that self antigens ARE indeed sometimes displayed on MHC2 as you say, this is rare though and should definitely not be an assumption we make off the bat. But the 2nd part cannot at all work, T cells just do not have MHC2. If it said what MHC class would macrophages display self antigens on (which was actually a question in one of my kaplan exams) then the answer could be MHC 1 and 2.

Also correct me if I'm wrong but Positive selection is simply the picking of the cells that are ABLE to recognize and react to appropriate antigens, while Negative selection is the weeding out of cells that react to self antigens. Both of these require the reading of another cell's antigens, not the display of own antigens (although this is happening, it is certainly not the point of selection).

Where did you see that T cells can express MHC2 if you don't mind me asking? I thought that MHC2 was limited to the 3 groups I mentioned above.

 

[PhilzCoffeeAddict]

I could be wrong here but I believe T cells don't DISPLAY MHC.. they recognize it. So for CD8 T cells, they recognize MHC1 and for CD4 T cells they recognize MHC2.

You're correct in positive and negative selection.

Honestly, you understand the concept. This is just a TERRIBLY worded question that you're thinking WAY too hard. AAMCs question are check thoroughly to be correct. You won't get a weirdly worded question like this.

 

[Ad2b]

AAMCs question are check thoroughly to be correct. You won't get a weirdly worded question like this.

And if you do, the answer will be based on a passage, not a discrete. The discretes are WAYYYYYYYY easier than this.

 

[Alpha-ketogluterate]

And if you do, the answer will be based on a passage, not a discrete. The discretes are WAYYYYYYYY easier than this.

Cool thanks guys, I was just trying to get to the bottom of this. While I really liked the EK books (although I give a slight edge to Kaplan in some categories), there are more than a handful of wrong questions and statements in each book. I think one of the worst ones was where it stated in both bold and red text that sodium potassium pumps are an example of secondary active transport (I think this is THE prime example of primary active transport, literally the example). And I know the author(s) know this, since later the text states the correct thing, just careless errors here and there. I still recommend them though. Anyways thanks for the help, I do not think this question is even right, not just poorly worded, but incorrect.

 

[aldol16]

Wait, T cells don't present antigens - they recognize antigens. Professional antigen-presenting cells, e.g. macrophages, CD11c+ dendritic cells, etc. display antigens. This question has to be wrong.

When negative selection is happening, single positive (i.e. CD4+ or CD8+) T cells try to bind antigen on epithelial cells in the thymus. Those epithelial cells are specialized to present antigens to T cells so that any T cells that react against self antigen can be negatively selected. Those that don't react (i.e. don't bind self-antigen) are released as mature T lymphocytes.