In virology, influenza A virus subtype H1N1 (A/H1N1) is a subtype of "Influenza A virus". Well known outbreaks of H1N1 strains in humans include the 2009 swine flu pandemic, the 1977 Russian flu pandemic as well as the 1918 flu pandemic. It is an orthomyxovirus that contains the glycoproteins haemagglutinin and neuraminidase. For this reason, they are described as H1N1, H1N2 etc. depending on the type of H or N antigens they express with metabolic synergy. Hemagglutinin causes red blood cells to clump together and binds the virus to the infected cell. Neuraminidase is a type of glycoside hydrolase enzyme which helps to move the virus particles through the infected cell and assist in budding from the host cells.