Hemagglutinin (HA) or haemagglutinin (BE) is an antigenic glycoprotein found on the surface of the influenza viruses (as well as many other bacteria and viruses). It is responsible for binding the virus to the cell that is being infected. The name "hemagglutinin" comes from the protein's ability to cause red blood cells (erythrocytes) to clump together ("agglutinate") in vitro [1]. http://www.wikidoc.org/index.php/Hemagglutinin

Influenza A Hemagglutinin 1 Neuraminidase 1

It basically means "type 5 hemagglutinin, type 1 neuraminidase". These are proteins expressed by the virus.

There are currently 17 known types of hemagglutinin and 9 known types of neuraminidase.

HN stands for hemagglutinin neuraminidase, a protein found in some viruses.

The viruses are identified by a series of letters and numbers that refer to two proteins, hemagglutinin (HA) and neuraminidase (NA).