Immunologic responses of children to serious infections with Streptococcus pneumoniae.

Antibody responses (as measured by radioimmunoassay), alterations in serum levels of complement, and the presence of circulating immune complexes (as measured by the fluid-phase C1Q-binding assay, the fluid-phase conglutinin assay, and the activation of C1 were evaluated in 15 children after meningitis and/or bacteremia caused by Streptococcus pneumoniae. The ages of the children ranged from two months to 16 years; the duration of follow-up ranged from 18 to 189 days (mean, 78 days). Increases in levels of homotypic antibody were found in only three (25%) of the 12 children in whom this response could be assessed, and all of these responses were transient. Eight (53%) of the 15 children had evidence of a heterotypic antibody response to S pneumoniae serotypes other than those causing their infections. The activation of C1 and C1q-binding activity were detected at the early stage of disease and were generally transient. The result of the fluid-phase conglutinin assay was positive for 14 (93%) of the 15 children later in the course of disease; this result was consistently positive throughout the follow-up period in the majority of children. Depressed concentrations of C4 were noted in nine children, depressed levels of C3 in four, and depressed levels of factor B in two.