These two proteins are a target of interest for antiviral drugs. Furthermore, they are also the antigen proteins to which a host antibodies can bind and trigger an immune response. Influenza type A viruses are categorized into different subtypes, or strains, based on which type of these two proteins is present on the surface of the virion. Currently, there are 16 subtypes of HA and 9 subtypes of NA known to exist. The most prevalent form of the different subtypes is H1N1. Single hemagglutinin-neuraminidase proteins, in which both HA and NA are found in a single protein, also exist. However, these will not be used in SensUs 2021.

The involvement of the hemagglutinin and neuraminidase proteins in the infection of H1N1 is essential for the virus reproduction, and a more specific mechanism is discussed below.

The hemagglutinin protein (HA) has the role of searching for the sialic acid receptors in respiratory-lining cell membranes. (figure _). Upon binding of this protein and the receptor, fusion of the virus and the cell membrane is facilitated with the help of glycan proteins. The virus then enters the cell where it sheds its shell and approaches the cell’s nucleus. Using the host replication mechanisms, the virus makes copies of itself. At this step of the infection process, important viral proteins are synthesized. These newly replicated viral elements subsequently attempt to leave through the cell membrane and infect other cells. To inhibit the exit of the viral components, sialic acid receptors on the cell membrane attempt to bind the HA glycoproteins. This is where viral evolution/mutability can play a role in the expansion of the capabilities of the virus.