Explain how the human immune system responds when a pathogen enters the body for the first time, and describe what happens if the same pathogen enters the body a second time.
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 receptors on their surface that bind to specific antigens on the pathogen. Lymphocytes then produce antibodies that are complementary to the antigen. The antibodies bind to the antigen, neutralising the pathogen or clumping it together (agglutination). Phagocytes such as macrophages then engulf and digest the pathogen (phagocytosis). After the first infection memory cells remain; if the same pathogen enters again, these memory cells trigger a faster antibody response and the pathogen is destroyed more quickly.
Examiner tips
- Use the exact terms: receptors, antigens, lymphocytes, antibodies, agglutination, phagocytosis, memory cells
- Show the sequence: detection → antibody production → neutralisation/agglutination → phagocytosis → memory
- Keep the answer concise – 5 points, one sentence per point
Common mistakes
- Confusing antibodies with T‑cell receptors
- Omitting the role of memory cells in the second response
- Using vague terms like "immune response" instead of specific mechanisms
Mark scheme (5 marks)
- White blood cells detect the pathogen using receptors on their cell surface to identify antigens on the pathogen
- Lymphocytes release antibodies that are specific / complementary to the antigen
- Antibodies bind to the antigens on the pathogen to neutralise it / clump pathogens together (agglutination)
- Phagocytes engulf / ingest and digest the pathogen (phagocytosis)
- Memory cells remain after the first infection so that if the same pathogen enters again, antibodies are produced more quickly / the pathogen is destroyed faster
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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