# Explain why a person who has recovered from one strain of influenza virus may still develop influenza when exposed to a different strain of the same virus.

> OCR A-Level Biology A (H420) — 4.1 Communicable diseases & immunity · Explain · 4 marks

> Influenza viruses undergo frequent changes to the proteins on their surface. Public health authorities issue updated vaccines each year to account for these changes.

## Mark scheme (4 marks)

1. Memory cells / antibodies produced are specific to the antigens of the original strain
2. The new strain has different surface antigens due to mutation / antigenic variation
3. Existing memory cells / antibodies cannot bind to / recognise the antigens of the new strain
4. A primary immune response must occur, so the virus replicates unchecked long enough to cause symptoms / disease

## Key terms

- [strain](https://www.gradenine.co.uk/glossary/strain)
- [influenza virus](https://www.gradenine.co.uk/glossary/influenza-virus)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-why-a-person-who-has-f3caf813) · Published by Druglandscape Ltd.