# Explain how antigenic variation in influenza viruses makes it difficult to develop long-lasting immunity in human populations.

> IB DP Biology Higher Level (2023 syllabus) — A2.3 Viruses (HL only) · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. 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.
2. 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.
3. 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.
4. 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

- [antigenic variation](https://www.gradenine.co.uk/glossary/antigenic-variation)

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