A rabbit was immunised with a heat-killed vaccine prepared from strain 1142 of The HA titres of the antisera against a surface-polysaccharide antigen extracted from the homologous strain were highest in the 2nd-4th weeks after immunisation and subsequently declined. Only a minor amount of 2-ME-resistant antibody was formed, and this at a late stage of the reaction. After a booster injection of the vaccine at the 10th week, there was a significantly greater HA-antibody response and a greater amount of 2-ME-resistant antibody was formed.

The relative passive-protective activity of the sera in mice corresponded to their content of 2-ME-sensitive HA antibody. The protective activity of the sera was 2-ME sensitive and was not removed by absorption with anti-rabbit IgG goat serum. Further, 280 μg of IgM-rich serum fraction obtained by sucrose-density-gradient ultracentrifugation passively protected against homologous challenge infection in mice, but even 910 μg of IgG-rich fraction did not. These results indicated that the protective antibody was of the IgM class.


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