Abstract

We determined the serum concentration of human IgO antibody to the native capsular polysaccharide of group B Streptococcus (GBS) type III needed to passively protect mice against lethal homologous challenge. Antibody was measured by an ELISA, standardized by two methods, and corrected for nonprecipitating antibody. A concentration of 1.3 µ of IgG antibody to GBS type III/mi protected 126 (97%) of 130 mice from an 80%–96% lethal dose bacterial challenge. Concentrations of IgG antibody to GBS type III in sera from 42 infected infants were ≤0.3 µ/ml. Concentrations of antibody ranged from <0.02 to 21.7 µ/ml in sera from 102 unselected pregnant women (median, 0.05 µ/ml); 13% had concentrations ≤1.3 µ/ml. Levels in 25 women colonized with GBS type III who gave birth to normal infants were significantly higher and ranged from 0.1 to 10.7 µ/ml (median, 0.78 µ/ml). In a study of transplacental passage of antibody, protective levels were found in a number of infants with gestational ages between 28 and 36 weeks.

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