Explain why a person who has recovered from one strain of influenza virus may still develop influenza when exposed to a different strain of the same virus.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Influenza viruses undergo frequent changes to the proteins on their surface. Public health authorities issue updated vaccines each year to account for these changes.
Model answer (4 marks)
Memory cells and antibodies produced during the first infection recognise only the specific surface antigens of that strain.
The new strain has altered surface proteins because of mutation/antigenic variation.
These altered antigens cannot be recognised or bound by the existing memory cells or antibodies.
Therefore a primary immune response is required, allowing the virus to replicate unchecked and cause disease.
The new strain has altered surface proteins because of mutation/antigenic variation.
These altered antigens cannot be recognised or bound by the existing memory cells or antibodies.
Therefore a primary immune response is required, allowing the virus to replicate unchecked and cause disease.
Examiner tips
- Use the term ‘memory cells’ and ‘antibodies’ to show understanding of adaptive immunity.
- Explain that antigenic variation changes surface proteins, preventing recognition.
- Mention that a primary response is needed, leading to symptoms.
- Keep the answer concise and use UK spelling.
Common mistakes
- Confusing the immune response as a secondary response; it is actually primary for the new strain.
- Omitting the role of antigenic variation in changing surface proteins.
- Using vague terms like ‘different virus’ instead of ‘different strain with altered antigens’.
Mark scheme (4 marks)
- Memory cells / antibodies produced are specific to the antigens of the original strain
- The new strain has different surface antigens due to mutation / antigenic variation
- Existing memory cells / antibodies cannot bind to / recognise the antigens of the new strain
- A primary immune response must occur, so the virus replicates unchecked long enough to cause symptoms / disease
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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