# Explain why a person who has recovered from one strain of influenza virus may still become ill 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 glycoprotein antigens on their surface. Two such processes are antigenic drift, involving small mutations, and antigenic shift, involving major reassortment of genetic material. Public health bodies must update influenza vaccines annually to match currently circulating strains.

## Mark scheme (4 marks)

1. Recovery from the first infection leads to the production of memory cells specific to the antigens of that strain
2. The new strain has different surface antigens due to mutation / antigenic variation
3. The existing memory cells / antibodies are not complementary to the new strain's antigens and so cannot bind to them
4. A new primary immune response must occur, which is slower, allowing the virus to replicate and cause symptoms before immunity is established

## 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)
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