# Explain how the human immune system responds when a pathogen enters the body for the first time, and describe what happens if the same pathogen enters the body a second time.

> Eduqas A-Level Biology — 3.1 Photosynthesis · Explain · 5 marks

## Mark scheme (5 marks)

1. White blood cells detect the pathogen using receptors on their cell surface to identify antigens on the pathogen
2. Lymphocytes release antibodies that are specific / complementary to the antigen
3. Antibodies bind to the antigens on the pathogen to neutralise it / clump pathogens together (agglutination)
4. Phagocytes engulf / ingest and digest the pathogen (phagocytosis)
5. Memory cells remain after the first infection so that if the same pathogen enters again, antibodies are produced more quickly / the pathogen is destroyed faster

## Related

- [Revision notes for Eduqas A-Level Biology](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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