Ontogeny of the humoral response to group A streptococcal carbohydrate: class and IgG subclass composition of antibodies in children.

We determined isotypes of natural antibodies to streptococcal group A carbohydrate (A-CHO) in sera from 101 children between 1 and 16 years of age, using a calibrated enzyme-linked immunosorbent assay system. Anti-A-CHO IgM could be detected in all but one sera. Median levels increased with age and were highest between 8 and 12 years. IgG antibodies were present at low concentrations up to the age of 4 years, and consisted predominantly of the IgG1 subclass. Between 4 and 8 years, concentrations of anti-A-CHO IgG markedly increased and median levels continued to increase through age 12-16. Anti-A-CHO IgG1 levels closely followed the pattern of IgG antibody concentrations. The number of IgG2 antibody positive sera was low in young children, as expected. In the 8-12 year age group and later, anti-A-CHO IgG2 was present in more than half of the samples, and in children between 12 and 16, medians of IgG2 and IgG1 antibodies were similar. Sera containing anti-A-CHO IgG3 were rare in children up to 4 years of age, but in the group of 4-8-year-old children, this subclass was detectable in 36% and later in up to 77% of the sera. Thus, the IgG response to A-CHO showed a clear maturation during childhood, involving the subclasses IgG1, IgG2 and IgG3. There were no significant differences in A-CHO levels between boys and girls.