There are three types of influenza viruses, types A, B and C, based on antigenic differences of their nucleo- and matrix proteins. Avian influenza viruses belong to type A. Influenza A and B viruses cause epidemics among human beings while influenza C causes only mild disease. For this reason, surveillance is usually performed for influenza A and B viruses only.
Influenza A viruses can be subdivided into subtypes based on the antigenicity of two different protein components, called antigens, on the surface of the virus. They are spike-like features called haemagglutinin (H) and neuraminidase (N) components. They are capable of eliciting subtype-specific immune responses which are fully protective within, but only partially protective across, different subtypes. Influenza A viruses currently cluster into sixteen H(H1-H16) and nine N (N1-N9) subtypes.
The H subtypes are epidemiologically most important, as they govern the ability of the virus to bind to specific receptor type(s) of a certain host and enter cells, where multiplication of the virus then occurs. The N subtypes govern the release of newly formed virus from the cells.