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Influenza Viruses and How They Change
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Influenza viruses have long been known for their ability to change, creating new strains of virus that infect naive populations.  Influenza viruses change by two processes, antigenic drift and antigenic shift.
Antigenic Drift
Antigenic drift is the process by which influenza viruses continously change their antigenic characteristics.  This is an important process driven by the immune response to influenza viruses.  Antibody targeted against the viral antigens bind and neutralize virus containing antigen.  Influenza viral DNA polymerase is prone to mistakes and new genes are often produced,  resulting in the production of new antigen.  The accumulation of these changes over time leads to mutations which produce hemagglutinin and neuraminidase that are not targeted by antibody. As a result new serotypes of the virus are produced.  Antigenic drift causes past immunity against influenza to become incapable in fighting the disease caused by mutated viral strains.
antigenic drift
Antigenic Shift
Antigenic shift is the formation of a new strain of virus by the combination of at least two different strains.  When a cell is simultaneously infected by two different strains of influenza virus the genes of each strain become mixed during replication.  As a result when the viral components recombine, some of the genetic material from one strain of virus ends up in the other, producing a new virus with new genetic material.  The influenza A genome allows the virus to easily undergo antigenic shift because it is segmented, allowing for more reassortment of the genetic material.  The new virus may be capable of infecting either of the species from which the first two strains originated.

antigenic shift
Swine as a "Mixing Vessel" For Influenza Viruses
Swine have long been considered to a be a "mixing vessel" for human and avian influenzas.  Hemagglutinin binds sialic acid on the receptor, allowing the virus to enter the cell (see pathogenesis).  Hemagglutinin on avian influenza has been shown to bind a different type of sialic acid than human influenza hemagglutinin.  Swine have both these types of sialic acid in their respiratory tracts2.  As a result, swine can be infected with either human or avian influenza facilitating antigenic shift.

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References
1. Ducatez M, Webster R, Webby R, 2008.  Animal influenza epidemiology.  Vaccine; 26S: D67-D69.
2. Thacker E, Janke B, 2008.  Swine influenza virus: Zoonotic potential and vaccination strategies for the control of avian and swine influenzas.  The Journal of Infectious Diseases; 197: S19-24