Hemagglutinin (designated as HA) and neuraminidase (designated as NA) are glycoproteins. Hemagglutinin and neuraminidase protrude from the outer surface of the influenza virus and neuraminidase is a constituent of the enveloping membrane that surrounds the viral contents. A glycoprotein is a protein that contains a short chain of sugar as part of its structure. The hemagglutinin and neuraminidase glycoproteins are important in the ability of the virus to cause influenza.

A typical influenza virus particle contains some 500 molecules of hemagglutinin and 100 molecules of neuraminidase. These are studded over the surface of the virus.

The illness caused by the influenza virus can be devastating. For example, in 1918 a new genetic variant of the virus swept around the world and in just over a year over 20 million people succumbed to the influenza. The variation was due to alterations in both the hemagglutinin...