thumb|300px|right|[[NIAID illustration of potential influenza genetic reassortment]]

Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is often applied specifically to influenza, as that is the best-known example, but the process is also known to occur with other viruses, such as visna virus in sheep. Antigenic shift is a specific case of reassortment or viral shift that confers a phenotypic change.

Antigenic shift is contrasted with antigenic drift, which is the natural mutation over time of known strains of influenza (or other things, in a more general sense) which may lead to a loss of immunity, or in vaccine mismatch. Antigenic drift occurs in all types of influenza including influenza A, influenza B and influenza C. Antigenic shift, however, occurs only in influenza A because it infects more than just humans. Affected species include other mammals and birds, giving influenza A the opportunity for a major reorganization of surface antigens. Influenza B and C principally infect humans, minimizing the chance that a reassortment will change its phenotype drastically.

In the 1940s, Maurice Hilleman discovered antigenic shift, which is important for the emergence of new viral pathogens as it is a pathway that viruses may follow to enter a new niche.

Role in the transmission of influenza viruses from non-human animals to people

Influenza A viruses are found in many different animals, including ducks, chickens, pigs, humans, whales, horses, and seals. Flu strains are named after their types of hemagglutinin and neuraminidase surface proteins (of which there are 18 and 9 respectively), so they will be called, for example, H3N2 for type-3 hemagglutinin and type-2 neuraminidase. Some strains of avian influenza (from which all other strains of influenza A are believed to stem The most recent 2009 H1N1 outbreak was a result of antigenic shift and reassortment between human, avian, and swine viruses.

Role of pigs in Influenza antigenic shift

Pigs are especially important in antigenic shift of influenza viruses. Because pigs can be infected with strains of influenza that infect various other species of animals, they act as 'mixing pots' for the virus. When multiple virus strains, such as a duck and human influenza strain, infect the same pig, antigenic shift is likely to occur. While most of the virus strains resulting from this will be dead-end strains, a few have the potential to become pandemic viruses.

See also

  • Coinfection
  • Antigenic variation

Notes

Further reading

  • Superflu: Antigenic shift in Influenza