Neuraminidase enzyme, molecular model. This enzyme is found on the surface of the influenza B virus. It plays a role in releasing the viruses from infected cells once they have used the cell's machinery to replicate. It is an antigenic protein, so triggers an immune response, and is made up of four subunits. Influenza B viruses can cause flu in humans and seals. 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.