Immunology: MHC class II deficiency: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Immunology: MHC class II deficiency

Sample item

A 9-month-old infant has chronic diarrhoea with Cryptosporidium, recurrent pneumonia and failure to thrive. CD4 T cells are markedly low with a normal CD8 count, immunoglobulins are low, and HLA-DR is absent on lymphocytes and monocytes. Which defect is most likely?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Failure of MHC class II expression due to mutation of regulatory transcription factors

Bare lymphocyte syndrome type II results from mutations in transcription factors such as CIITA, RFXANK, RFX5 or RFXAP that drive MHC class II gene expression. Without class II molecules, CD4 T cells are not positively selected in the thymus and antigen presentation to helper cells fails, so antibody production is poor. Class I expression is normal.

Option by option

  • A. Failure of V(D)J recombination due to RAG mutation

    RAG mutation prevents both B and T cell receptor formation and leaves HLA-DR present.

  • B. Failure of MHC class II expression due to mutation of regulatory transcription factors (correct)

    Absent class II molecules explain the selective CD4 deficiency and absent HLA-DR.

  • C. Failure of T-cell receptor signalling due to absent CD3

    CD3 deficiency impairs T-cell receptor signalling but does not remove HLA-DR expression.

  • D. Failure of MHC class I expression due to TAP mutation

    TAP deficiency (type I) reduces class I expression and causes low CD8 cells with relatively preserved CD4 cells.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Failure of MHC class II expression due to mutation of regulatory transcription factors. Bare lymphocyte syndrome type II results from mutations in transcription factors such as CIITA, RFXANK, RFX5 or RFXAP that drive MHC class II gene expression.

Why is "Failure of V(D)J recombination due to RAG mutation" not the answer?

RAG mutation prevents both B and T cell receptor formation and leaves HLA-DR present.

Why is "Failure of T-cell receptor signalling due to absent CD3" not the answer?

CD3 deficiency impairs T-cell receptor signalling but does not remove HLA-DR expression.

Why is "Failure of MHC class I expression due to TAP mutation" not the answer?

TAP deficiency (type I) reduces class I expression and causes low CD8 cells with relatively preserved CD4 cells.

What concept does this question test?

It tests Immunology: MHC class II deficiency in Microbiology. Without class II molecules, CD4 T cells are not positively selected in the thymus and antigen presentation to helper cells fails, so antibody production is poor. Class I expression is normal.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Immunology: MHC class II deficiency?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.