Explain how the human immune system responds when a pathogen enters the body for the first time, and why a second infection by the same pathogen is overcome more quickly.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. White blood cells recognise antigens on the surface of the invading pathogen.
2. Lymphocytes (B‑cells) produce antibodies that are complementary to those antigens.
3. The antibodies bind to the pathogen, neutralising it or causing agglutination so it can be cleared more easily.
4. Phagocytes such as macrophages engulf and digest the antibody‑coated pathogen (phagocytosis).
5. After the first infection memory B‑cells persist; on re‑exposure they produce antibodies rapidly and in larger amounts, eliminating the pathogen before symptoms develop.
2. Lymphocytes (B‑cells) produce antibodies that are complementary to those antigens.
3. The antibodies bind to the pathogen, neutralising it or causing agglutination so it can be cleared more easily.
4. Phagocytes such as macrophages engulf and digest the antibody‑coated pathogen (phagocytosis).
5. After the first infection memory B‑cells persist; on re‑exposure they produce antibodies rapidly and in larger amounts, eliminating the pathogen before symptoms develop.
Examiner tips
- Use the exact terms – white blood cells, lymphocytes, antibodies, phagocytes, memory cells. Show the sequence: detection → antibody production → neutralisation/agglutination → phagocytosis → memory response. Include the word ‘first infection’ and ‘second infection’ to match the question wording.
Common mistakes
- Mixing up T‑cells with B‑cells or forgetting that antibodies are produced by B‑cells. Failing to mention memory cells or the speed/quantity advantage in the second response. Using vague phrases like ‘immune response’ without detailing the steps.
Mark scheme (5 marks)
- White blood cells detect antigens on the surface of the pathogen
- Lymphocytes release/produce antibodies that are specific/complementary to that antigen
- Antibodies bind to antigens on the pathogen to neutralise it / cause agglutination making it easier to destroy
- Phagocytes engulf/ingest and digest the pathogen (phagocytosis)
- Memory cells remain after the first infection so that on second exposure antibodies are produced more rapidly/in greater numbers, destroying the pathogen before symptoms appear
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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