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 quickly.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Phagocytes recognise and engulf the invading pathogen by phagocytosis, destroying it.
Lymphocytes (B‑cells) then recognise the pathogen’s antigen and produce specific antibodies.
During this primary response memory B‑cells are generated.
If the same pathogen enters again, the memory cells recognise its antigen immediately.
They activate rapidly, producing large amounts of antibody quickly, which neutralises the pathogen before symptoms appear, so the second infection is dealt with more swiftly.
Lymphocytes (B‑cells) then recognise the pathogen’s antigen and produce specific antibodies.
During this primary response memory B‑cells are generated.
If the same pathogen enters again, the memory cells recognise its antigen immediately.
They activate rapidly, producing large amounts of antibody quickly, which neutralises the pathogen before symptoms appear, so the second infection is dealt with more swiftly.
Examiner tips
- Use the word ‘phagocytosis’ and mention phagocytes; link to antibody production; state memory cell formation; explain rapid response on re‑infection; show why symptoms are avoided.
- Include all five points in a logical order; keep sentences concise; use correct terminology (B‑cells, antibodies, antigen).
Common mistakes
- Omitting the role of phagocytes or the term ‘phagocytosis’.
- Failing to mention memory cells or the rapid antibody response on re‑infection.
Mark scheme (5 marks)
- Phagocytes engulf and destroy the pathogen (by phagocytosis)
- Lymphocytes produce antibodies that are complementary to / specific to the antigen on the pathogen
- Memory cells are produced during the first infection
- On second infection, memory cells recognise the same antigen and respond more quickly / rapidly
- More antibodies are produced more rapidly so the pathogen is destroyed before symptoms develop / the person does not become ill
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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