The above image shows the different features of an influenza virus, including the main surface proteins, hemagglutinin (HA) and neuraminidase (NA). Following influenza virus infection or receipt of a flu vaccine, the body’s immune system develops antibodies that recognize and bind to "antigenic sites," which are regions found on an influenza virus’ surface proteins. By binding to the HA antigenic sites, antibodies can neutralize influenza viruses, which prevents them from causing further infection. (Antibodies that bind to the NA antigenic sites can also reduce further spread.)