# Chlamydia is a communicable disease caused by the bacterium Chlamydia. Explain how the body's defences may prevent the Chlamydia bacterium from causing infection, and describe what happens if the bacterium does enter the body and trigger an immune response.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 5.2 Pathogens and infection · Explain · 5 marks

> Chlamydia is one of the most common sexually transmitted infections in the UK. It is caused by the bacterium Chlamydia and is spread through sexual contact. Many infected individuals show no symptoms, meaning the disease can be passed on unknowingly.

## Mark scheme (5 marks)

1. The skin / mucous membranes act as a physical barrier that prevents the bacterium from entering the body
2. If bacteria enter the body, white blood cells / phagocytes engulf and destroy them (phagocytosis)
3. The bacterium carries antigens on its surface that are recognised as foreign / non-self
4. B-lymphocytes produce antibodies specific to the antigen on the Chlamydia bacterium
5. Antibodies bind to the antigens on the bacterium, leading to its destruction / neutralisation / clumping so it can be engulfed

## Key terms

- [communicable disease](https://www.gradenine.co.uk/glossary/communicable-disease)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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/chlamydia-is-a-communicable-disease-caused-c4758e0e) · Published by Druglandscape Ltd.