Explain how antigenic variation in influenza viruses makes it difficult to develop long-lasting immunity in human populations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Influenza viruses possess surface glycoproteins — haemagglutinin (HA) and neuraminidase (NA) — that are recognised by the host immune system. These proteins are subject to frequent change through two distinct processes: antigenic drift and antigenic shift.
Model answer (4 marks)
Antigenic drift causes small point mutations in the HA or NA genes, giving slightly altered surface proteins that existing antibodies may not recognise.
Antigenic shift results from reassortment of whole RNA segments when two strains co‑infect a cell, creating a novel HA and/or NA that the immune system has never seen.
Because the surface antigens change, memory B cells and antibodies from a previous infection or vaccination may fail to bind the new strain, so the adaptive response must be rebuilt.
Consequently, influenza vaccines must be reformulated each year, as the circulating strains differ from those in the previous vaccine, preventing long‑lasting immunity.
Antigenic shift results from reassortment of whole RNA segments when two strains co‑infect a cell, creating a novel HA and/or NA that the immune system has never seen.
Because the surface antigens change, memory B cells and antibodies from a previous infection or vaccination may fail to bind the new strain, so the adaptive response must be rebuilt.
Consequently, influenza vaccines must be reformulated each year, as the circulating strains differ from those in the previous vaccine, preventing long‑lasting immunity.
Examiner tips
- Use the terms ‘antigenic drift’ and ‘antigenic shift’ explicitly. Show the link between antigen change and loss of antibody recognition. Explain the need for annual vaccine reformulation. Keep each point concise to fit the 4‑mark structure.
Common mistakes
- Confusing drift with shift or vice versa. Failing to mention the impact on memory B cells. Omitting the annual vaccine reformulation requirement.
Mark scheme (4 marks)
- Antigenic drift involves the accumulation of point mutations in genes encoding HA or NA, producing slightly altered surface proteins that may no longer be recognised by existing antibodies.
- Antigenic shift involves the reassortment of entire RNA genome segments when two different influenza strains co-infect the same cell, producing a novel combination of HA and/or NA that the immune system has not previously encountered.
- Because the surface antigens change, memory B cells and antibodies produced in response to a previous infection or vaccination may no longer recognise the new viral strain, so the adaptive immune response must be mounted largely from scratch.
- This antigenic variation requires annual reformulation of influenza vaccines because strains circulating each season may differ from those in the previous year's vaccine, meaning no single vaccine confers durable, broad protection.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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