Immunology: natural killer cells: 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: natural killer cells

Sample item

A virus down-regulates MHC class I expression on infected cells to escape cytotoxic T cells. Which cell is best placed to kill such cells, and why?

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

The answer and the reasoning

C. Natural killer cells, because inhibitory receptors are not engaged by missing MHC class I

NK cells survey for the missing-self signal. Normal cells display MHC class I that engages inhibitory KIR or CD94/NKG2A receptors. When class I is lost, inhibition is released and activating signals trigger perforin and granzyme-mediated killing, covering a gap left by CD8 T cells.

Option by option

  • A. CD4 helper T cells, because they recognise MHC class I directly

    CD4 cells recognise peptides on MHC class II, not class I.

  • B. Plasma cells, because they release perforin

    Plasma cells secrete antibody and do not kill by perforin.

  • C. Natural killer cells, because inhibitory receptors are not engaged by missing MHC class I (correct)

    Correct. Lack of inhibitory MHC I signalling activates NK killing.

  • D. CD8 cytotoxic T cells, because they respond to absent MHC

    CD8 cells require MHC class I to recognise peptide, so they are blinded by its loss.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Natural killer cells, because inhibitory receptors are not engaged by missing MHC class I. NK cells survey for the missing-self signal.

Why is "CD4 helper T cells, because they recognise MHC class I directly" not the answer?

CD4 cells recognise peptides on MHC class II, not class I.

Why is "Plasma cells, because they release perforin" not the answer?

Plasma cells secrete antibody and do not kill by perforin.

Why is "CD8 cytotoxic T cells, because they respond to absent MHC" not the answer?

CD8 cells require MHC class I to recognise peptide, so they are blinded by its loss.

What concept does this question test?

It tests Immunology: natural killer cells in Microbiology. Normal cells display MHC class I that engages inhibitory KIR or CD94/NKG2A receptors. When class I is lost, inhibition is released and activating signals trigger perforin and granzyme-mediated killing, covering a gap left by CD8 T cells.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: natural killer cells?

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.