Haemagglutinin protein from the surface of Influenza A virus, molecular model. This protein, known as haemagglutinin, is found on the surface of the influenza A virus. It is responsible for binding the virus to the cell membrane before it enters the cell. There are approximately 500 haemaglutinin molecules on the surface of each influenza A virus. Depending upon the strain of the virus, it can cause human, bird (avian), swine, horse or dog flu. In this model, solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices; arrows are beta sheets), and the faded tubular structure behind represents the individual atoms and bonds.