ABSTRACT
Influenza type A viruses undergoes two quite distinct kinds of antigenic variation, antigenic drift and major antigenic shifts. As the human population becomes immune to infection by extant strains of influenza virus, so the pressure rises to select variants which, by displaying small but significant changes in antigenicity, can evade the immune response. There are two antigens on the surface of the influenza virus particle, the hemagglutinin (HA) and the neuraminidase (NA). Antibody to the HA neutralizes the infectivity of the virus and therefore variation in the HA is of greater importance than variation in the NA. Antigenic variants of the A/PR8 and A/Hong Kong/68 strains of influenza virus were isolated after a single passage of these viruses in the presence of monoclonal hybridoma antibodies to the hemagglutinin. As in the case of type A influenza, each variant was not inhibited in HI tests by the monoclonal antibodies used in its selection.
