Antigenic shift

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 influenzavirus A, influenza B and influenza C. Antigenic shift, however, occurs only in influenzavirus 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.

Antigenic shift is important for the emergence of new viral pathogens as it is a pathway that viruses may follow to enter a new niche. It could occur with primate viruses and may be a factor for the appearance of new viruses in the human species such as HIV. Due to the structure of its genome, HIV does not undergo reassortment, but it does recombine freely and via superinfection HIV can produce recombinant HIV strains that differ significantly from their ancestors.

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