<P> Hemagglutinin (HA) and neuraminidase (NA) are the two large glycoproteins on the outside of the viral particles . HA is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell, while NA is involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles . Thus, these proteins are targets for antiviral drugs . Furthermore, they are antigens to which antibodies can be raised . Influenza A viruses are classified into subtypes based on antibody responses to HA and NA . These different types of HA and NA form the basis of the H and N distinctions in, for example, H5N1 . There are 16 H and 9 N subtypes known, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans . </P> <P> Viruses can replicate only in living cells . Influenza infection and replication is a multi-step process: First, the virus has to bind to and enter the cell, then deliver its genome to a site where it can produce new copies of viral proteins and RNA, assemble these components into new viral particles, and, last, exit the host cell . </P> <P> Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells, typically in the nose, throat, and lungs of mammals, and intestines of birds (Stage 1 in infection figure). After the hemagglutinin is cleaved by a protease, the cell imports the virus by endocytosis . </P> <P> The intracellular details are still being elucidated . It is known that virions converge to the microtubule organizing center, interact with acidic endosomes and finally enter the target endosomes for genome release . </P>

Where does the influenza virus settle in your body