Explain why a person who has recovered from one strain of influenza virus may still become infected by a different strain of the same virus.

OCR A-Level Biology A (H420) — 4.1 Communicable diseases & immunity · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Influenza viruses exist as multiple strains, each capable of causing respiratory disease in humans. Vaccination programmes must be updated annually to account for changes in circulating strains.

Model answer (4 marks)

Memory cells and antibodies produced after the first infection recognise only the surface antigens of that particular strain. Influenza viruses mutate, giving new strains with altered surface antigens. The memory cells/antibodies cannot bind to the new antigens, so they cannot neutralise the virus. Consequently a primary immune response is required, which takes time and allows the new strain to cause infection and disease.

Examiner tips

  • Use the word ‘memory cells’ and ‘antibodies’ to show specific immune components.
  • Explain that antigenic variation changes surface proteins, preventing cross‑reactivity.
  • Show the sequence: specific immunity → antigenic change → no binding → primary response required.
  • Keep the answer concise – 4 points, one sentence each.

Common mistakes

  • Confusing the immune response with the virus itself; e.g. saying the virus is ‘immune’ rather than the host’s immune system.
  • Failing to mention that the new strain’s antigens are different, or that the existing antibodies cannot bind them.
  • Using vague terms like ‘different’ without specifying ‘surface antigens’ or ‘antigenic drift’.

Mark scheme (4 marks)

  1. Memory cells / antibodies produced after first infection are specific to the antigens of the original strain
  2. Different strains of influenza have different surface antigens
  3. Existing memory cells / antibodies cannot bind to the new strain's antigens
  4. A primary immune response must occur, meaning the new strain is not eliminated quickly enough to prevent disease / infection

Key terms in this question

strain · influenza

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