Mechanism of antigenic shift in influenza: What process generates major antigenic changes in influenza viruses leading to pandemics?

Introduction / Context:
Influenza epidemics and pandemics are driven by antigenic evolution. Two mechanisms are classically tested: antigenic drift (point mutations) and antigenic shift (major changes).

Given Data / Assumptions:

• Influenza genome is segmented negative-sense ssRNA (8 segments).
• Antigenic shift causes abrupt, major antigenic changes.

Concept / Approach:
Antigenic shift arises when two different influenza viruses co-infect the same cell and exchange genome segments (reassortment). This can create novel hemagglutinin and/or neuraminidase combinations to which human populations lack immunity, precipitating pandemics.

Step-by-Step Solution:
Identify the unique property enabling shift: segmented genome. Describe event: co-infection and segment reassortment. Outcome: major antigenic novelty in HA/NA leading to pandemics. Select reassortment as the mechanism.

Verification / Alternative check:
Historical pandemics (e.g., 1957, 1968, 2009) show genomic evidence of segment reassortment events.

Why Other Options Are Wrong:

• Somatic recombination of antibody chains: describes B-cell processes, not viral evolution.
• Capsule type expression: relevant to bacteria, not influenza viruses.
• Receptor canyon changes: a concept used in picornaviruses, not the primary mechanism of influenza shift.

Common Pitfalls:
Confusing antigenic drift (point mutations) with shift (reassortment). Both occur, but only shift produces large, abrupt changes.

Final Answer:
Reassortment of RNA segments from different influenza viruses