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[Antigenic structure of the hemagglutinin of H1N1 influenza viruses studied by radioimmunological analysis].

To elucidate the antigenic structure of influenza A (H1N1) viruses of different circulation periods (1947-1979) the criterion of participation of viruses of a constant test-group in competition for antibody from hyperimmune rabbit sera in radioimmunoassay (RIA) was used. The "set" of competing viruses varied with each immunospecific serum in relation to the immobilized strain for which the competitive capacity was studied. Each definite "set" of competing viruses was used as a marker of a certain type of antigenic determinant detected with a given test serum. Analysis of distribution of antigenic determinants and nonimmunogenic areas of hemagglutinin in the strains tested showed the drift nature of changes of the hemagglutinin antigenic properties. These changes may be grouped in 4 large antigenic regions, A, B, C, and D which appear to correspond to the "antigenic sites" revealed by Gerhard et al. by the monoclonal antibody procedure. Antigenic changes occur mainly in 3 of the above regions: A, B, and C in viruses of the first circulation period, and in B, C, and D in viruses of the second circulation period. Thus it has been shown that analysis of the antigenic structure of hemagglutinin by reciprocal radioimmunoassay of immunospecific sera to strains of the same subtype may be used as a tool for antigenic site mapping.