Describe how the human body responds to a pathogen that has entered the bloodstream, including the roles of lymphocytes, antibodies and phagocytes.

WJEC A-Level Biology (Wales) — 3.2 Respiration · 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 specific to those antigens. The antibodies bind to the pathogen, neutralising it or causing agglutination, which clumps the foreign cells together. Phagocytes engulf and digest the pathogen through phagocytosis, using enzymes to break it down. After the infection, memory cells remain, allowing a faster and stronger response if the same pathogen re‑enters the body.

Examiner tips

  • Use the exact terms: white blood cells, lymphocytes, antibodies, phagocytes, memory cells.
  • Show the sequence: detection → antibody release → binding/neutralisation → phagocytosis → memory.
  • Keep the answer concise – 5 points, one sentence per point.
  • Mention that antibodies are complementary to the antigen.

Mark scheme (5 marks)

  1. White blood cells detect the pathogen by identifying antigens on its surface
  2. Lymphocytes release antibodies that are specific / complementary to the antigen
  3. Antibodies bind to antigens on the pathogen to neutralise it / cause agglutination (clumping together of foreign cells)
  4. Phagocytes engulf / ingest and digest the pathogen using enzymes (phagocytosis)
  5. Memory cells remain after infection so that the same pathogen can be destroyed more quickly if it enters the body again

Key terms in this question

pathogen · lymphocyte · phagocyte

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