Can you survive this question?

D) Recognition of self MHC molecules

My ans is D) Recognition of self MHC molecules

MHC restriction and maturation/selection of developing T-cells recognize foreign antigens in association with only the self MHC molecules and not in association with foreign MHC molecules.

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No body survived!!!!!

Nobody survived! me too! I chose D. I was scratching my head when i read explanation. How could i not know positive and negative selection? They are a few things that this Q reminds us.

First, how much we try, we are never going to get 100 out of 100.
Test makers know what we read and try to be one step ahead of us.
Always select the best answer. They may be two choices that seem correct, but you have to select one of the best.

The correct answer is B. I chose this Q from WebPath immunology Section. Here comes their explanation!

B. CORRECT. The progenitor cells in the thymus need to express both types of receptors in order to make it to the next step in development. Later they will differentiate further to become either a CD4 or CD8 cell.

Stem cells are first doubly -ve for CD4 and CD and for TCR as well, then doubly positive for CD4 and CD8 and for TCR as well. If they don't express CD4 and CD8, they can not make it to the next step in proper thymic education. They must be mature enough to undergo education. If they are not, they die. Next cells that don't recognize self MHC die; they are useless. they won't present any antigens. Why give them nutrition and let them live, while they do nothing good! Then, cells that recognize self MHC are allowed to live. But those that present self antigens or those that react strongly with MHC are killed as well. They are dangerous and will kill our own. finally, we are left with cells that are harmless and useful, sensible and true patroits. Now we call that real education!!!!

Choice D comes a little later in education.

There is even one more Q on the same topic, with some variation.

In a study of normal embryogenesis, it is shown that proliferation and maturation of lymphocytes occurs. Many pre-lymphocytes are observed to undergo apoptosis within the thymus. Which of the following mechanisms is most likely to induce apoptosis in these pre-lymphocytes?

A Binding to macrophages in the bone marrow
B Circulation through the thymic cortex
C Failure to express surface antigen receptor
D Recognition of self MHC molecules
E Recombination of V, D, and J genes

C. CORRECT. Those lymphocytes not expressing surface antigen receptor will be of no value, for they will not recognize antigens, and so they can be safely eliminated in development.

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