# Explain how natural killer (NK) cells recognise and destroy virus-infected host cells without prior sensitisation to a specific antigen.

> IB DP Biology Higher Level (2023 syllabus) — C3.2 Defence against disease · Explain · 4 marks

## Mark scheme (4 marks)

1. NK cells continuously monitor host cells for the presence of MHC class I molecules on their surface
2. Virus-infected cells down-regulate / reduce expression of MHC class I molecules, so NK cells recognise the 'missing self' signal
3. NK cells release perforin, which forms pores / channels in the target cell membrane
4. NK cells also release granzymes that enter through the pores and trigger apoptosis in the infected cell, preventing viral replication

## Key terms

- [natural killer (NK) cell](https://www.gradenine.co.uk/glossary/natural-killer-nk-cell)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-natural-killer-nk-cells-92e032cb) · Published by Druglandscape Ltd.