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 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. Influenza A viruses are found in many different animals, including ducks, chickens, pigs, humans, whales, horses, and seals. Influenza B viruses circulate widely principally among humans, though it has recently been found in seals.