Explain how the immune system responds when a pathogen enters the body for the first time and why a second infection by the same pathogen is dealt with more rapidly.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a pathogen enters the body, the immune system mounts a specific response involving white blood cells. If the same pathogen enters the body again, the immune response is faster and stronger.
Model answer (5 marks)
The pathogen carries surface antigens that are recognised as foreign by the immune system.
B‑lymphocytes that recognise the antigen are activated and produce antibodies specific to that antigen.
The antibodies bind to the pathogen, neutralising it or marking it for destruction by other cells.
During the first infection a small number of activated B‑cells become memory cells.
On a second encounter with the same pathogen, the memory cells are rapidly re‑activated and produce large amounts of the specific antibodies, so the pathogen is destroyed before symptoms appear.
B‑lymphocytes that recognise the antigen are activated and produce antibodies specific to that antigen.
The antibodies bind to the pathogen, neutralising it or marking it for destruction by other cells.
During the first infection a small number of activated B‑cells become memory cells.
On a second encounter with the same pathogen, the memory cells are rapidly re‑activated and produce large amounts of the specific antibodies, so the pathogen is destroyed before symptoms appear.
Examiner tips
- Use the exact terms ‘antigen’, ‘B‑lymphocyte’, ‘antibody’, ‘memory cell’.
- Show the sequence: recognition → antibody production → memory cell formation → rapid response.
- Mention that the second response is faster because memory cells are already primed.
Common mistakes
- Confusing T‑cells with B‑cells or omitting the role of antibodies.
- Failing to state that memory cells are produced during the first infection.
- Using vague phrases like ‘immune system’ without specifying the cells involved.
Mark scheme (5 marks)
- The pathogen has antigens on its surface that are recognised as foreign
- B-lymphocytes produce antibodies specific to that antigen
- The antibodies bind to / destroy the pathogen, neutralising it or marking it for destruction
- Memory cells are produced during the first infection
- On second infection, memory cells rapidly produce large quantities of the correct antibodies, so the pathogen is destroyed before symptoms develop
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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