# A child receives a vaccination containing a weakened form of a pathogen. Several weeks later, the child is exposed to the same pathogen in its virulent form but does not develop the disease. Explain why the child does not become ill following this second exposure.

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

## Mark scheme (4 marks)

1. The vaccination caused clonal expansion of specific B lymphocytes / T lymphocytes that recognised the antigen on the weakened pathogen
2. Memory cells (B memory / T memory cells) were produced and persist long-term after the initial vaccination
3. On re-exposure to the virulent pathogen, memory cells are rapidly activated / undergo clonal expansion much faster than the primary response
4. Antibodies are produced in large quantities before the pathogen can replicate to a level that causes symptoms / the pathogen is destroyed before disease develops

## Key terms

- [vaccination](https://www.gradenine.co.uk/glossary/vaccination)
- [virulent](https://www.gradenine.co.uk/glossary/virulent)
- [pathogen](https://www.gradenine.co.uk/glossary/pathogen)

## 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/a-child-receives-a-vaccination-containing-f8c5217c) · Published by Druglandscape Ltd.