Opsonic activity of human IgG and IgM antibody for type III group B streptococci.

We separated IgG and IgM from 10 adult sera containing measurable (greater than or equal to 0.05 micrograms/mL) antibody of both isotypes to the type III polysaccharide of group B streptococci. Ig preparations were used to preopsonize 3H-labeled type III group B streptococci which were added to adherent monolayers of human, monocyte-derived macrophages. Reproducibly significant phagocytosis, measurable by accumulation of radioactivity and confirmed by electron microscopy, required both antibody (from adult serum or Ig preparation) and complement (from cord serum deficient in specific antibody). All adult IgG and IgM preparations were opsonic in the presence of complement. When a cord serum containing IgG but no IgM antibody was separated by the same procedure, the IgG fraction was opsonic but the fraction that would contain IgM was not. The opsonic activity of IgM in adult sera persisted after the removal of contaminating IgG4. These observations indicate that both IgG and IgM antibody in adult sera are opsonic for type III GBS and support the hypothesis that IgM deficiency in newborns may be a contributing factor to certain bacterial infections.