Antigenic shift is the process by which two different strains of influenza combine to form a new subtype having a mixture of the surface antigens of the two original strains. The term antigenic shift is specific to the influenza literature; in other viral systems, the same process is called reassortment or viral shift. Negatively stained flu virions. ... Reassortment is the exchange of DNA between viruses inside a host cell. ... A viral shift is the widespread emergence of a seemingly novel virus. ...

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) to evade the immune system. Antigenic drift occurs in all types of influenza including influenza A, B and 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 only infect humans, minimizing the chance to mutate drastically. Antigenic drift refers to mutations in the influenza virus over time. ... Orders Subclass Multituberculata (extinct) Plagiaulacida Cimolodonta Subclass Palaeoryctoides (extinct) Subclass Triconodonta (extinct) Subclass Eutheria (includes extinct ancestors)/Placentalia (excludes extinct ancestors) Afrosoricida Artiodactyla Carnivora Cetacea Chiroptera Cimolesta (extinct) Creodonta (extinct) Condylarthra (extinct) Dermoptera Desmostylia (extinct) Embrithopoda (extinct) Hyracoidea Insectivora Lagomorpha Litopterna (extinct) Macroscelidea Mesonychia (extinct) Notoungulata (extinct) Perissodactyla Pholidota Plesiadapiformes... Orders Many - see section below. ...

Flu strains are named after their types of hemagglutinin and neuraminidase surface proteins, so they will be called, for example, H3N2 for type-3 hemagglutinin and type-2 neuraminidase. If two different strains of influenza infect the same cell simultaneously, their protein capsids and lipid envelopes are removed, exposing their RNA, which is then transcribed to mRNA. The host cell then forms new viruses that combine antigens; for example, H3N2 and H5N1 can form H5N2 this way. Because the human immune system has difficulty recognizing the new influenza strain, it may be highly dangerous. Such combinations caused, for instance, the infamous Spanish Flu outbreak of 1918 which killed 40 million people worldwide. Influenza virus which have undergone antigenic shift have also gone on to cause the Asian Flu pandemic of 1957, the Hong Kong Flu pandemic of 1968, and the Swine Flu scare of 1976. One increasingly worrying situation is the possible antigenic shift between avian influenza and human influenza. This antigenic shift could cause the formation of a highly virulent virus. Hemagglutinin, as depicted in a simplified molecular model. ... Neuraminidase ribbon diagram Neuraminidase is an antigenic glycoprotein enzyme (EC 3. ... The outer shell of a virus is called the capsid. ... Ribonucleic acid (RNA) is a nucleic acid polymer consisting of covalently bound nucleotides. ... The interaction of mRNA in a eukaryote cell. ... The immune system is the system of specialised cells and organs that protect an organism from outside biological influences. ... The Spanish Flu Pandemic, also known as La Grippe, was an unusually severe and deadly strain of avian influenza, a viral infectious disease, that killed some 25 million to 50 million people worldwide in 1918 and 1919. ... The Avian Flu was a pandemic outbreak of influenza that originated in China in 1957 and spread worldwide that same year. ... The Hong Kong Flu was a pandemic outbreak of influenza that began in Hong Kong in 1968 and spread to the United States of America that year. ... Swine Flu is a form of Type A influenza that is normally virulent only in pigs. ...

Transmission of Influenza Viruses from Animals to People

Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. Influenza B viruses circulate widely only among humans.

There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes, all of which have been found among influenza A viruses in wild birds. Wild birds are the primary natural reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza A viruses in all other animals. Most influenza viruses cause asymptomatic or mild infection in birds; however, the range of symptoms in birds varies greatly depending on the strain of virus. Infection with certain avian influenza A viruses (for example, some strains of H5 and H7 viruses) can cause widespread disease and death among some species of wild and especially domestic birds such as chickens and turkeys. Hemagglutinin, as depicted in a simplified molecular model. ... Neuraminidase ribbon diagram Neuraminidase is an antigenic glycoprotein enzyme (EC 3. ...

Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs get symptoms similar to humans, such as cough, fever, and runny nose. Because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus.

For example, if a pig were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could mix (reassort) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus. The resulting new virus would likely be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans. This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person to person, an influenza pandemic can occur. Hemagglutinin, as depicted in a simplified molecular model. ... Neuraminidase ribbon diagram Neuraminidase is an antigenic glycoprotein enzyme (EC 3. ... Hemagglutinin, as depicted in a simplified molecular model. ... Neuraminidase ribbon diagram Neuraminidase is an antigenic glycoprotein enzyme (EC 3. ...

In 2004, scientists pointed out that the avian influenza virus might undergo an antigenic shift with the human flu virus and cause a global influenza pandemic like the one in 1918. 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... Influenza A virus, the virus that causes Avian flu. ... A pandemic, or global epidemic, is an outbreak of an infectious disease that affects people over an extensive geographical area (from Greek pan all + demos people). ... 1918 (MCMXVIII) was a common year starting on Tuesday of the Gregorian calendar (see link for calendar) or a common year starting on Wednesday of the Julian calendar. ...

External links

• An illustration of antigenic shift