Describe how the human immune system responds when a pathogen enters the body for the first time, and explain why a second infection by the same pathogen is dealt with more quickly.

Eduqas A-Level Biology — 3.3 Microbiology · Describe and Explain · 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 antigens on the pathogen’s surface. Lymphocytes then release antibodies that are complementary to the antigen; the antibodies bind to and neutralise the pathogen. Phagocytes engulf and digest the pathogen using enzymes (phagocytosis). After the first infection, memory cells remain in the body. On a second infection, these memory cells rapidly divide and produce the correct antibodies much more quickly, destroying the pathogen before symptoms develop.

Examiner tips

  • Use the word "recognise" for detection, "release" for antibody production, and "engulf" for phagocytosis.
  • Mention memory cells and rapid antibody production to justify the faster response.
  • Keep the answer concise – one sentence per point.
  • Use UK spelling (recognise, realise).

Common mistakes

  • Confusing antibodies with T‑cells or failing to mention memory cells.
  • Using “immune system” instead of specifying white blood cells, lymphocytes and phagocytes.
  • Omitting the rapid division of memory cells or the quicker antibody response on re‑infection.

Mark scheme (5 marks)

  1. White blood cells detect antigens on the surface of the pathogen
  2. Lymphocytes release antibodies that are complementary/specific to the antigen, which bind to and neutralise the pathogen
  3. Phagocytes engulf and digest the pathogen using enzymes (phagocytosis)
  4. After the first infection, memory cells remain in the body
  5. On second infection, memory cells rapidly divide and produce the correct antibodies much more quickly, destroying the pathogen before symptoms develop

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