Opsonic specificity of human antibody to the type III polysaccharide of group B Streptococcus.

The functional significance of vaccine-induced or naturally acquired antibody to the capsular polysaccharide of type III group B Streptococcus was determined in human sera by means of an opsonophagocytic assay. Sera containing a sufficient concentration of type-specific antibody and endogenous serum complement caused a significant reduction (greater than 1 log10) in colony-forming units for each of 22 type III clinical isolates after incubation for 40 min. In contrast, serum with a very low concentration of antibody (0.4 micrograms/ml) killed only one isolate. Absorption of serum containing vaccine-induced antibody with purified native type III polysaccharide decreased opsonophagocytosis by a mean of 1.2 log10 for 11 selected type III strains. These results indicate a lack of major antigenic differences among a wide spectrum of type III strains, since bactericidal activity of human serum is uniformly observed in the presence of complement and sufficient concentrations of antibody to the type III polysaccharide.