Human IgG Antibody to Group B Streptococcus Type III: Comparison of Protective Levels in a Murine Model with Levels in Infected Human Neonates

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