A person receives a vaccine against measles. Some years later, the same person is exposed to the measles virus. Explain why the person does not develop the disease.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Vaccines contain antigens that stimulate the immune system without causing disease. Memory cells produced after vaccination can persist in the body for many years.
Model answer (5 marks)
The vaccine introduces antigens from the measles virus into the body.
White blood cells (lymphocytes) recognise these antigens and produce antibodies that are specific to the measles antigen.
During the primary response, memory B‑cells and T‑cells are generated.
When the person is later exposed to the measles virus, the memory cells recognise the same antigen.
They respond rapidly, producing large amounts of antibodies that neutralise the virus before symptoms develop, so the disease does not occur.
White blood cells (lymphocytes) recognise these antigens and produce antibodies that are specific to the measles antigen.
During the primary response, memory B‑cells and T‑cells are generated.
When the person is later exposed to the measles virus, the memory cells recognise the same antigen.
They respond rapidly, producing large amounts of antibodies that neutralise the virus before symptoms develop, so the disease does not occur.
Examiner tips
- Use the term ‘memory cells’ and explain their rapid response on re‑exposure; link antibody production to prevention of symptoms.
- Show the sequence: antigen → antibody production → memory cell formation → rapid secondary response.
- Mention that the secondary response is faster and stronger than the primary.
- Use the word ‘specific’ when describing antibodies and antigens.
Common mistakes
- Confusing the primary and secondary immune responses; forgetting to mention memory cells.
- Using vague terms like ‘immune system’ without specifying lymphocytes or antibodies.
- Failing to state that the secondary response is faster and more effective, leading to no disease.
Mark scheme (5 marks)
- The vaccine introduces antigens (from the measles virus) into the body
- White blood cells (lymphocytes) produce antibodies specific to the measles antigen
- Memory cells are produced after the initial vaccination
- On re-exposure to the measles virus, memory cells recognise the antigen and respond rapidly
- Antibodies are produced in large quantities quickly enough to destroy the virus before symptoms develop / disease occurs
Key terms in this question
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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