Mutations help the virus to survive through small or significant changes in its antigenic sites. Most viruses mutate gradually over time to improve survival. Influenza viruses can mutate through antigenic shifts and antigenic drifts.
Antigenic drift is typical in both type A and type B. The HA (hemagglutinin) and NA (neuraminidase) proteins are the primary sites for antibody recognition. The RNA segments that encode these proteins mutate slightly so that they are less recognizable by the antibodies.
Antigenic shifts are when whole segments of RNA have been reassorted. Shifts only occur in influenza type A. This phenomenon occurs when two types of influenza A are circulating in the body simultaneously. The shifts usually happen in areas where humans and fowl or swine live close to each other. Most people are not immune when these shifts happen, and a pandemic occurs.
