Radioimmunoassay of influenza A virus haemagglutinin. II. Antigenic cross-reactions of influenza A (H3 subtype) viruses as determined by radioimmunoassay and haemagglutination inhibition tests.

Individual rabbits differed greatly in their antibody response to the "strain-specific" and "cross-reactive" antigenic determinants on the haemagglutinin (HA) subunit of influenza virus recombinant MRC11 (H3N2) and influenza virus Dunedin (H3N2), after immunization with whole virus or bromelain-released haemagglutinin (B-HA). Consequently, diverse cross-reactions between htese viruses and A/Hong Kong/68 virus were found in the haemagglutination inhibition (HI) test as well as in homologous radioimmunoassay (125I-B-HA from MRC11:anti MRC11 serum, and 125I-B-HA from Dunedin: anti Dunedin serum) when sera from different animals were employed. Radioimmunoassay (RIA), over and above to the HI test, was able to differentiate clearly the respective HAs also with antisera reacting to the same HI titre with both corresponding influenza virus strains. Thus it appeared that antigenic differences could be identified with higher sensitivity by homologous RIA than by the HI test and that multiple antigenic determinants were reactive on the 125I-B-HA in the RIA procedure employed. MRC11 and A/HK/68 viruses were also compared by heterologous RIA (125I-B-HA from MRC11: anti A/HK/68 serum). It was found that preferentially antigenic determinants with a high degree of cross-reactivity could be studied in the heterologous system.