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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
The skin and mucous membranes form a physical barrier that prevents Chlamydia from entering the body.
If the bacterium breaches this barrier, white blood cells (phagocytes) engulf and destroy it by phagocytosis.
Chlamydia displays surface antigens that are recognised as foreign by the immune system.
B‑lymphocytes produce antibodies that are specific to these antigens.
The antibodies bind to the bacterial antigens, neutralising the bacterium, causing clumping and making it easier for phagocytes to engulf and destroy it.
If the bacterium breaches this barrier, white blood cells (phagocytes) engulf and destroy it by phagocytosis.
Chlamydia displays surface antigens that are recognised as foreign by the immune system.
B‑lymphocytes produce antibodies that are specific to these antigens.
The antibodies bind to the bacterial antigens, neutralising the bacterium, causing clumping and making it easier for phagocytes to engulf and destroy it.
Examiner tips
- Use the exact terms: skin/mucous membranes, phagocytes, antigens, B‑lymphocytes, antibodies, neutralisation/clumping.
- Show the sequence from barrier to antibody action to get all 5 marks.
Common mistakes
- Confusing the skin with the immune response; forgetting to mention phagocytosis; using vague terms like "immune cells" instead of "white blood cells".
Mark scheme (5 marks)
- The skin / mucous membranes act as a physical barrier that prevents the bacterium from entering the body
- If bacteria enter the body, white blood cells / phagocytes engulf and destroy them (phagocytosis)
- The bacterium carries antigens on its surface that are recognised as foreign / non-self
- B-lymphocytes produce antibodies specific to the antigen on the Chlamydia bacterium
- Antibodies bind to the antigens on the bacterium, leading to its destruction / neutralisation / clumping so it can be engulfed
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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