Quantitation of serum antibodies to surface antigens of group B streptococci types Ia, Ib, and III: low antibody levels in mothers of neonatally infected infants.

A method has been developed for the detection in human sera of antibodies to surface antigens of group B streptococci (GBS), types Ia, Ib, or III. The sera were absorbed with GBS type II and then added to a suspension of GBS of the type against which antibodies were to be measured. After incubation and washing of the bacteria, the antibodies present on the surface of the cells were quantitated with radiolabelled protein A. Antibodies of IgG class were detected, probably specific for types Ia, Ib, or III, with the exception that the Ib GBS suspension used could detect some antibodies to type Ia. With this method it was found that 6/7 mothers to infants with GBS septicemia had low levels of serum antibodies to the infant's type of GBS. Urogenital carriers of GBS, giving birth to neonatally healthy infants, had higher serum antibody levels against the colonizing type.