A child receives a vaccination containing a weakened form of a pathogen. Several weeks later, the child is exposed to the same pathogen in its virulent form but does not develop the disease. Explain why the child does not become ill following this second exposure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The weakened pathogen in the vaccine stimulates a primary immune response, causing clonal expansion of specific B and T lymphocytes that recognise its antigens.
Memory B and T cells are generated and persist long‑term.
When the child is later exposed to the virulent strain, these memory cells are rapidly re‑activated and undergo a secondary clonal expansion.
Large amounts of specific antibodies are produced quickly, neutralising the pathogen before it can replicate to a level that causes disease.
Memory B and T cells are generated and persist long‑term.
When the child is later exposed to the virulent strain, these memory cells are rapidly re‑activated and undergo a secondary clonal expansion.
Large amounts of specific antibodies are produced quickly, neutralising the pathogen before it can replicate to a level that causes disease.
Examiner tips
- Use the term ‘memory cells’ and explain their rapid response on re‑exposure.
- Show the sequence: primary response → memory cell formation → secondary rapid response.
- Mention that antibodies are produced in large quantities before symptoms appear.
Common mistakes
- Confusing the vaccine with the virulent strain; the vaccine is weakened. Failing to mention memory cells or the rapid secondary response. Using vague terms like ‘immune system’ without specifying B/T cells or antibodies.
Mark scheme (4 marks)
- The vaccination caused clonal expansion of specific B lymphocytes / T lymphocytes that recognised the antigen on the weakened pathogen
- Memory cells (B memory / T memory cells) were produced and persist long-term after the initial vaccination
- On re-exposure to the virulent pathogen, memory cells are rapidly activated / undergo clonal expansion much faster than the primary response
- Antibodies are produced in large quantities before the pathogen can replicate to a level that causes symptoms / the pathogen is destroyed before disease develops
Key terms in this question
vaccination · virulent · pathogen
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Communicable diseases & immunity questions
- A student claims that active immunity and passive immunity both provide long-ter…
- Explain why a person who has recovered from one strain of influenza virus may st…
- Explain why a person who has recovered from one strain of influenza virus may st…
- Explain why a person who has recovered from one strain of influenza virus may st…