Bacterial Cultures Form Flower Patterns (IMAGE) University of Arizona Caption 3D computer-generated rendering of a whole influenza (flu) virus. Our immune response is triggered by the virus' hemagglutinin (HA) and neuraminidase (NA) surface proteins, shown in semi-transparent blue. HA is a trimer comprised of three subunits, while NA is a tetramer, comprised of four subunits, with a head region resembling a 4-leaf clover. Credit Illustration by Dan Higgins, courtesy of CDC/ Douglas Jordan Usage Restrictions This visual may only be used in conjunction with reporting or posting this news release. Credit must be given as indicated. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.