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Hemagglutinin (influenza)
Influenza hemagglutinin (HA) or haemagglutinin (British English) is a glycoprotein found on the surface of influenza viruses. It is responsible for binding the virus to cells with sialic acid on the membranes, such as cells in the upper respiratory tract or erythrocytes. It is also responsible for the fusion of the viral envelope with the endosome membrane, after the pH has been reduced. The name "hemagglutinin" comes from the protein's ability to cause red blood cells (erythrocytes) to clump together ("agglutinate") ''in vitro''.
==Subtypes==
HA has at least 18 different antigens. These subtypes are named H1 through H18. H16 was discovered in 2004 on influenza A viruses isolated from black-headed gulls from Sweden and Norway. H17 was discovered in 2012 in fruit bats.〔Unique new flu virus found in bats http://www.nhs.uk/news/2012/03march/Pages/cdc-finds-h17-bat-influenza.aspx〕 Most recently, H18 was discovered in a Peruvian bat in 2013. The first three hemagglutinins, H1, H2, and H3, are found in human influenza viruses.
Viral neuraminidase (NA) is another protein found on the surface of influenza. Influenza viruses are characterized by the type of HA and NA that they carry; hence H1N1, H5N2 etc.
A highly pathogenic avian flu virus of H5N1 type has been found to infect humans at a low rate. It has been reported that single amino acid changes in this avian virus strain's type H5 hemagglutinin have been found in human patients that "can significantly alter receptor specificity of avian H5N1 viruses, providing them with an ability to bind to receptors optimal for human influenza viruses". This finding seems to explain how an H5N1 virus that normally does not infect humans can mutate and become able to efficiently infect human cells. The hemagglutinin of the H5N1 virus has been associated with the high pathogenicity of this flu virus strain, apparently due to its ease of conversion to an active form by proteolysis.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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