text: The Influenza virus is an RNA virus that is divided into three serological types: A, B and C. Hemagglutinin (HA) and neuraminidase (NA) are two important glycoproteins on influenza virus membranes. The hemagglutinin is a sialic acid receptor-binding molecule and mediates entry of the virus into the host cell, while neuraminidase cleaves cellular-receptor sialic acid to form new particles. Neuraminidase is an exoglycosidase that destroys the hemagglutinin receptor by cleaving the α(2,6)- or α(2,3)-ketosidic linkage that exists between a terminal sialic acid and a sugar residue of the Neu5Ac containing receptor on the surface of host cells. This helps the spread of the infection by preventing self-aggregation of new viruses at the cell surface and possible immobilisation in the mucin by hemagglutinin (HA) during virus replication. The virus will then be released from the host cells and will subsequently infect other cells. Neuraminidase also helps the invasion of the virus in the upper respiratory tract, possibly by cleaving sialic acid molecules on mucin of epithelial cells. Neuraminidase is found in influenza viruses of types A and B. Neuraminidase has roles in the infection, replication and delivery of Influenza virus A and B. Type C Influenza virus expresses the enzyme esterase instead of neuraminidase.