Explain how natural killer (NK) cells recognise and destroy virus-infected host cells without prior sensitisation to a specific antigen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
NK cells constantly survey host cells for MHC class I molecules. Virus‑infected cells often down‑regulate MHC I, creating a "missing‑self" signal that activates NK cells. The activated NK cell releases perforin, forming pores in the target membrane, and secretes granzymes that enter through the pores to trigger apoptosis, thereby destroying the infected cell and stopping viral replication.
Examiner tips
- Mention "missing‑self" signal first to show understanding of recognition mechanism.
- Explain both perforin and granzyme actions to cover the full process.
- Use terms "MHC class I", "perforin", "granzymes", "apoptosis" exactly as in the mark scheme.
- Keep answer concise – 4 points only.
Mark scheme (4 marks)
- NK cells continuously monitor host cells for the presence of MHC class I molecules on their surface
- Virus-infected cells down-regulate / reduce expression of MHC class I molecules, so NK cells recognise the 'missing self' signal
- NK cells release perforin, which forms pores / channels in the target cell membrane
- NK cells also release granzymes that enter through the pores and trigger apoptosis in the infected cell, preventing viral replication
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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