Chlamydia is a communicable disease caused by a bacterium. Explain how the human body attempts to defend itself against a Chlamydia infection, from the point at which the bacterium first enters the body to the point at which it is destroyed.

Eduqas A-Level Biology — 3.5 Human impact on the environment · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

1. The skin and mucus membranes act as a first line of defence, preventing the bacterium from entering the body.
2. Once inside, white blood cells recognise the Chlamydia bacterium through receptors that bind to antigens on its surface.
3. Lymphocytes then produce antibodies that are complementary to these antigens.
4. The antibodies bind to the bacterial antigens, neutralising the pathogen or causing agglutination, which makes it easier for immune cells to recognise and destroy the bacteria.
5. Phagocytes engulf the antibody‑coated bacteria and digest them using enzymes (phagocytosis).

Examiner tips

  • Use the exact terminology: skin/mucus, receptors, antigens, antibodies, agglutination, phagocytosis. Structure answer in five clear points matching the mark scheme.

Mark scheme (5 marks)

  1. Non-specific defence: skin / mucus / chemical barriers act as a first line of defence to prevent the pathogen entering the body
  2. White blood cells detect the bacterium using receptors that identify antigens on the surface of the pathogen
  3. Lymphocytes produce antibodies that are specific / complementary to the antigen on the Chlamydia bacterium
  4. Antibodies bind to the antigens on the bacterium to neutralise / disable it, or clump bacteria together (agglutination) to make destruction easier
  5. Phagocytes engulf and digest the bacterium / pathogen using enzymes (phagocytosis)

Key terms in this question

communicable disease

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