Development of IgM antibody to group B Streptococcus type III in human infants

An ELISA was developed to measure IgM antibody to albumin-coupled native capsular polysaccharide of type III group B streptococcus (GBS). The assay was standardized by two double-label methods that agreed within 33%. In quantitative assays, the range of IgM antibody to type III GBS in the sera of 94 adult pregnant women was 1.2-50.6 micrograms/ml (median, 5.4), while each of 38 cord serum samples contained less than 0.03 micrograms/ml IgM antibody. Neonatal rats were passively immunized with a serially diluted human serum containing 15 micrograms/ml IgM and undetectable IgG antibody to type III GBS. The rats were protected against lethal infection with an IgM antibody concentration of 0.5 micrograms/ml. In single serum samples from 31 healthy infants less than 2 years old and serial specimens from 5 infants with type III GBS infections, specific IgM antibody was detectable by 3 months of age. Levels greater than or equal to 0.5 micrograms/ml were present in all samples from infants greater than 7 months of age. The acquisition of specific IgM antibody is inversely correlated with the age-limited incidence of type III GBS infections in young children.