# Explain how the human immune system responds when a pathogen enters the body for the first time, and why a second infection by the same pathogen is overcome more quickly.

> WJEC A-Level Biology (Wales) — 3.3 Microbiology · Explain · 5 marks

## Mark scheme (5 marks)

1. White blood cells detect antigens on the surface of the pathogen
2. Lymphocytes release/produce antibodies that are specific/complementary to that antigen
3. Antibodies bind to antigens on the pathogen to neutralise it / cause agglutination making it easier to destroy
4. Phagocytes engulf/ingest and digest the pathogen (phagocytosis)
5. Memory cells remain after the first infection so that on second exposure antibodies are produced more rapidly/in greater numbers, destroying the pathogen before symptoms appear

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](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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