Class-specific antibody response to Haemophilus influenzae type b capsular polysaccharide vaccine.

Bacteremic infections caused by H influenzae type b have their peak incidence in children around one year of age, at a time when the serum antibody response to the capsular polysaccharide is very low [1, 3]. The capsular polysaccharide vaccine has in fact been shown to be efficacious in preventing the disease above but not below the age of 18 months [1]. Preliminary observations supported the notion that slow maturation of cells that synthesize antibody to H influenzae type b altered immunoglobulin class distribution of the response and resulted specifically in a defective IgG antibody response [1]. We have used a solid-phase ELISA to measure the class-specific antibody responses in 14 infants and children vaccinated with the capsular polysaccharide vaccine. We found that, among the infants who were less than 18 months old and whose antibody response was generally low, the response was only IgG-specific in two children and only IgM-specific in two others. In one child an IgG and IgM response was detected, and in two no response was detected. Among the older children, all children showed an increase in IgG antibody; only two also had an increase of IgM antibody. IgA antibodies were not detectable in any preimmune sera. The first IgA antibody responses were seen at the age of 15 months but were common thereafter. These data do not indicate a specific defect of IgG-class antibodies in the antibody response to the capsular polysaccharide vaccine.