H and N are two distinct proteins found on the surface of the influenza virus. H stands for hemagglutinin and N stands for neuraminidase. They have very exact shapes that can bind, like a key in its lock, with places on the surface of cells in the respiratory tract. There are 16 different H proteins and 9 different N proteins, which means there are 144 possible HN combinations. The subtypes of type A influenza viruses are named according to these H and N protein combinations. For example, H5N1 refers to a subtype with hemagglutinin (H) type 5 and neuraminidase (N) type 1. The particular strain of avian influenza virus that has infected poultry, some wild birds and a number of people in other parts of the world is the highly pathogenic subtype H5N1.
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