Correlation of serum immunoglobulin subclass concentrations with antibody responses of children to immunization with Haemophilus influenzae type b polysaccharide-pertussis vaccine

Children less than 24 months of age respond poorly to immunization with the capsular polysaccharide of Haemophilus influenzae type b. Because human antibodies to polysaccharide antigens are relatively restricted to IgG₂, a late-maturing subclass, we examined the relationship between serum subclass concentrations and anticapsular antibody responses of 41 healthy children, 9 to 38 months of age, following immunization with type b polysaccharide mixed with pertussis vaccine. Both total and IgG anticapsular antibody responses correlated significantly with preimmune serum concentrations of IgG₂ but not with those if IgG₁. This correlation was age dependent, however, and after the effect of age was removed by partial correlation, the correlation between anticapsular antibody responses and serum IgG₂ concentrations was no longer significant. These findings indicate that the ability to respond to this vaccine coincides with maturation of the ability to secrete immunoglobulin of the IgG₂ subclass; however, individual variation in IgG₂ that is independent of age does not correlate with antibody response to the type b polysaccharide.