Explain why a person who has recovered from one strain of influenza virus may still become ill when exposed to 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 undergo frequent changes to the glycoprotein antigens on their surface. Two such processes are antigenic drift, involving small mutations, and antigenic shift, involving major reassortment of genetic material. Public health bodies must update influenza vaccines annually to match currently circulating strains.

Model answer (4 marks)

Recovery from the first infection produces memory B and T cells that recognise the specific surface antigens of that strain.
The new strain has altered glycoprotein antigens because of antigenic drift or shift.
These altered antigens are not recognised by the existing memory cells or antibodies, so they cannot bind and neutralise the virus.
Consequently a new primary immune response is required, which takes longer to develop, allowing the virus to replicate and cause symptoms before immunity is established.

Examiner tips

  • Use the term ‘memory cells’ and ‘antibodies’ to show understanding of adaptive immunity.
  • Explain that the new strain’s antigens differ, so existing immunity is ineffective.
  • Mention the slower primary response and its consequence for symptoms.

Common mistakes

  • Confusing the immune response as a secondary one; it is a primary response to the new strain.
  • Omitting that the altered antigens are due to mutation or reassortment.
  • Failing to state that the existing memory cells cannot bind the new antigens.

Mark scheme (4 marks)

  1. Recovery from the first infection leads to the production of memory cells specific to the antigens of that strain
  2. The new strain has different surface antigens due to mutation / antigenic variation
  3. The existing memory cells / antibodies are not complementary to the new strain's antigens and so cannot bind to them
  4. A new primary immune response must occur, which is slower, allowing the virus to replicate and cause symptoms before immunity is established

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