Flu virus surface protein and antibody. Molecular model of the neuraminidase glycoprotein enzyme found on the surface of the influenza (flu) virus, in complex with the antigen-binding fragment (Fab) of monoclonal antibody NC41. The presence of neuraminidase on the virus surface enables the virus to be released from the host cell. It is also a target for monoclonal antibody drugs that bind to it (seen here) and prevent influenza infection.