Influenza viruses are members of the family Orthomyxoviridae and are divided into three genres, A, B, and C. Influenza A and B viruses are most relevant clinically, since they cause severe respiratory infections in humans. Influenza A (H1N1), A (H3N2), and one or two influenza B viruses (depending on the vaccine) are included in each year’s influenza vaccine.
In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from the human influenza A (H1N1) viruses circulating at that time. The new virus caused the first influenza pandemic in more than 40 years. That virus (often called “2009 H1N1”) has now replaced the H1N1 virus that was previously circulating in humans.
|HA1||The hemagglutinin (HA) is the principal antigen in inactivated influenza vaccines and the target of protective antibodies. HA is a major envelope glycoprotein with a length of approximately 560 amino acid residues, which is subsequently cleaved into two subunits, HA1 and HA2.The HA1 subunit forms a membrane-distal globular head that contains the receptor-binding site and most of the highly variable immunodominant antigenic regions recognized by neutralizing antibodies.|
|M||The integral membrane protein (M), forms a tetramer with ion channel activity. M is involved in the infection process by modulating the pH within virions. M is the target for the anti-influenza drugs.|
|NA||The neuraminidase (NA) is envelope glycoprotein, and is involved in the process of new virions budding out of host cells. NA has enzymatic activity, and is the target for the antiviral drugs, thus slowing down the release of progeny virus from infected cells.|