* H: Hemagglutinin

* N: Neuraminidase

These are two of the major surface proteins found on influenza viruses. They are responsible for:

* Hemagglutinin (H): This protein helps the virus attach to and infect cells. It binds to sialic acid receptors on the surface of cells, allowing the virus to enter.

* Neuraminidase (N): This protein helps the virus release newly formed viruses from infected cells. It breaks down sialic acid molecules, preventing the virus from becoming stuck to the cell.

The specific type of H and N protein present on a virus determines its subtype. For example, the influenza A virus responsible for the 1918 pandemic was H1N1. There are currently 18 different H subtypes and 11 different N subtypes identified.

Understanding the roles of these proteins is crucial for developing antiviral drugs and vaccines against influenza.