How do hemagglutinin and neuraminidase help influenza viruses infect cells?

Hemagglutinin and neuraminidase are essential surface proteins that facilitate the influenza virus life cycle through distinct mechanisms:

• Hemagglutinin enables the virus to initiate infection by binding to sialic acid on the surface of host cells. This attachment is a critical first step that allows the virus to enter and infect the host organism.1
• Neuraminidase acts as an enzyme that cleaves sialic acid molecules. After the virus replicates inside a host cell and begins to bud off, it remains tethered to the cell surface via the bond between hemagglutinin and sialic acid. Neuraminidase breaks this connection, effectively freeing the newly formed virus particles so they can spread and infect other cells.2

Together, these proteins are vital for the virus's ability to propagate within a host. Because they are highly susceptible to genetic mutations, they are also the primary targets for the host's immune system and are used to classify influenza A subtypes, such as H1N1.21