Explain how the human immune system destroys a pathogen after it has entered the body. Include in your answer the roles of lymphocytes, antibodies and phagocytes.

WJEC A-Level Biology (Wales) — 3.2 Respiration · 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 a pathogen by detecting antigens on its surface.
Lymphocytes are then activated and produce antibodies that are complementary to the antigen.
The antibodies bind to the antigen, neutralising the pathogen or causing agglutination.
Phagocytes recognise the antibody‑coated pathogen and engulf it.
Inside the phagocyte the pathogen is destroyed by enzymes in lysosomes, rendering it harmless.

Examiner tips

  • Use the exact terms: white blood cells, lymphocytes, antibodies, phagocytes. Show the sequence: detection → antibody production → binding/neutralisation → phagocytosis → digestion. Include the word ‘antigen’ and ‘enzymes’ to hit all points.
  • common_mistakes
  • :
  • Mixing up lymphocytes with other white blood cells. Forgetting to mention the role of antibodies. Using vague terms like ‘immune cells’ instead of specific cell types.

Mark scheme (5 marks)

  1. White blood cells detect pathogens by identifying antigens on the pathogen's surface
  2. Lymphocytes trigger the release of antibodies that are complementary/specific to the antigen
  3. Antibodies bind to the antigens on the pathogen to neutralise it / antibodies cause agglutination (clumping together) of pathogens
  4. Phagocytes ingest/engulf the pathogen
  5. Phagocytes digest the pathogen using enzymes, making it harmless

Key terms in this question

lymphocyte · phagocyte

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