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Discussion Starter · #1 ·
In the thymus during fetal life, most of the T lymphocytes are observed to undergo apoptosis, leaving a population of functional cells that form the basis for cell mediated immunity. Which of the following characteristics is most likely to insure survival of T cells in the thymus?

A Absence of a complete beta chain
B CD4 and CD8 receptor expression
C Presence of CD3
D Recognition of self MHC molecules
E Strong MHC peptide recognition
 

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A Absence of a complete beta chain - Part of T Cell Receptor hence its absence would not be conducive for survival.
B CD4 and CD8 receptor expression - This is what they have in the cortex area, for them to survive the medullary selection and go ahead will mean to lose on of them.
C Presence of CD3 - I think its a T cell identifier - not exactly sure why i would eliminate this option but have a feeling it is not the right option.
D Recognition of self MHC molecules - This is beneficial so we won't have an autoimmune reaction.
E Strong MHC peptide recognition - The moment it says strong means its reacting too much which is not what we want.
 

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Discussion Starter · #9 ·
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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guys m confused .. see both B and D are correct as we know .. and question stem asks us MOST LIKELY mechanism of non selection of T cell in thymus.. And MOST of the times T cells are not selected because they fail in positive selection .. I have done similar question before and for that it was D... now can anybody plz help:eek::eek:
 

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Discussion Starter · #17 ·
guys m confused .. see both B and D are correct as we know .. and question stem asks us MOST LIKELY mechanism of non selection of T cell in thymus.. And MOST of the times T cells are not selected because they fail in positive selection .. I have done similar question before and for that it was D... now can anybody plz help:eek::eek:
There are multiple stages in the development of a T cell. This Q is simply asking which stage comes first and causes MOST cells to die. You have to pay attention to the wording of the Q. They may make a Q with the same choices, but the answer next time may be D.
 
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