Describe how the human immune system responds when a pathogen enters the body for the first time, and explain why the response is faster if the same pathogen enters the body a second time.

Eduqas A-Level Biology — 3.4 Population size and ecosystems · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

White blood cells recognise the pathogen by detecting antigens on its surface.
Lymphocytes then release antibodies that are complementary to those antigens.
Phagocytes ingest and digest the pathogen through phagocytosis.
After the infection, some lymphocytes become memory cells and remain in the body.
If the same pathogen enters again, memory cells rapidly divide and produce antibodies more quickly, destroying the pathogen before symptoms appear.

Examiner tips

  • Use the exact terminology: white blood cells, lymphocytes, antibodies, phagocytes, memory cells. Show the sequence of events in the correct order. Explain the role of memory cells in the faster second response.
  • common_mistakes
  • :
  • Confusing antibodies with antigens. Forgetting to mention memory cells. Describing the response as ‘faster’ without explaining why (memory cells).

Mark scheme (5 marks)

  1. White blood cells detect the pathogen using antigens on its surface
  2. Lymphocytes release antibodies that are complementary / specific to the antigen
  3. Phagocytes ingest and digest the pathogen (phagocytosis)
  4. After the infection, memory cells remain in the body
  5. On second exposure, memory cells rapidly divide and produce antibodies more quickly, destroying the pathogen before symptoms appear

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