Analyses of the antigenicity of influenza haemagglutinin at the pH optimum for virus-mediated membrane fusion.

At the pH optimum for membrane fusion the haemagglutinin glycoprotein (HA) of the influenza virus membrane which is implicated in the fusion activity undergoes a conformational change. We have analysed the effects of this change on the antigenicity of the haemagglutinin by reacting the molecule with monoclonal antibodies of defined specificity. The results obtained indicate that specific changes in antigenicity occur in antigenic sites B and D and are interpreted in terms of the three-dimensional structure of the molecule and the effects of low pH incubation on it. Our results also provide evidence for the antigenic significance of amino acid sequence changes in site B of the HAs of natural isolates and allow clear delineation of this site into two regions.