All 110 rabbits immunized with Group A, A-variant, and C streptococcal vaccines produced 19S anti-IgG in addition to antibodies to the streptococcal carbohydrates. 19S anti-IgG was detected by hemagglutination of rabbit red blood cells coated with rabbit anti-blood group F antibody. Antisera of 88 of these animals were also tested for 7S anti-IgG with a coprecipitation assay. This assay is based on the coprecipitation of 7S anti-IgG with complexes of streptococcal carbohydrate and anti-carbohydrate antibody. 50 of the 88 anti-Group C streptococcal antisera contained 7S anti-IgGs. In eight antisera the concentration was greater than 5 mg/ml.

The data suggest a genetic influence on the occurrence of 7S anti-IgG. The eight rabbits which produced more than 5 mg/ml of 7S anti-IgG belonged to three related families. Moreover, there were families in which almost every member produced 7S anti-IgG and other families in which only 30% of the members manufactured 7S anti-IgG.

The streptococcal vaccine was an especially efficient stimulus for the production of 19S anti-IgG, whereas the pneumococcal vaccine was much less effective in this respect. Furthermore, 7S anti-IgGs were not detected in antipneumococcal antisera, although the concentration of anti-capsular antibodies was similar to that of anti-carbohydrate antibodies in antistreptococcal antisera.

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