Sialic acid
To infect a host cell, the influenza virus attaches to the exterior cell surface using hemagglutinin, a molecule found on the surface of the virus that binds to sialic acid groups. Sialic acids are found on various glycoproteins at the host cell surface. The virus then moves from sialic acid group to sialic acid group until it finds the proper cell surface receptor (whose identity remains unknown). Neuraminadase enables this movement by cleaving sialic acid groups that hemagglutinin was attached to. After the virus has entered the cell and has replicated, new viral particles bud from the host cell membrane. The hemagglutinin on new viral particles remains attached to sialic acid groups of glycoproteins on the external cell surface and the surface of other viral particles; neuraminadase cleaves these groups and thereby allows the release of viral particles and prevents self-aggregation. Neuraminadase also facilitates the movement of virus particles in the presence of mucus rich in silicic acid.