Estimation of the protective level of human IgG antibody to the type-specific polysaccharide of group B Streptococcus type Ia.

Human IgG antibody to the type-specific polysaccharide antigen of group B Streptococcus type Ia in the sera of mice was measured by enzyme-linked immunosorbent assay, previously standardized by quantitative precipitation, 24 hr after passive immunization with human serum or affinity-chromatographed antibody. The concentrations of antibody needed to protect mice against 90% lethal dose challenge varied with the bacterial inoculum and ranged from 0.25 to 1 microgram/ml using five strains of group B Streptococcus type Ia. Affinity-chromatographed antibody gave results comparable to serum, indicating the specificity of the antibody and the absence of other humoral factors in protection with this serum. Sera from 11 infected infants and their mothers had concentrations of antibody of less than or equal to 0.17 micrograms/ml, below the protective level delineated in the experimental model. Twelve percent of 50 adult women and 36% of 25 women colonized with group B Streptococcus type Ia had antibody levels of greater than or equal to 1 microgram/ml.