Immunoglobulin G and M composition of naturally occurring antibody to type III group B streptococci

Human sera were examined by an enzyme-linked immunosorbent assay for immunoglobulin G (IgG) and IgM antibodies to purified type III polysaccharide of group B streptococci. The antigen-binding capacity of a reference human serum was determined by a radioimmunoassay, and the total antibody content was determined by quantitative precipitation. The serum was then depleted of IgM and IgA to determine the effect on the antigen-binding capacity. Duplicate samples of 81 sera were tested by the enzyme-linked assay in comparison with reference standard serum. Although levels of IgG antibody were greater in subjects who had carried type III streptococci during pregnancy, concentrations of this antibody were generally low. Only 2 of 28 sera (7%) from parturient subjects and 7 of 25 sera (28%) from adult volunteers contained greater than or equal to 1 microgram of IgG antibody per ml; the mean levels were 0.13 and 0.53 micrograms/ml, respectively. In contrast, 19 of 28 maternal sera (68%) and 22 of 25 (88%) volunteer adult sera contained greater than or equal to 1 microgram/ml of IgM antibody; mean levels were 1.33 and 1.54 micrograms/ml, respectively. The cord serum levels of IgG antibody were almost identical to maternal serum concentrations, whereas IgM antibody was essentially undetected.