Antibody responses in invasive group B streptococcal infection in adults.

Nonpregnant adults with group B streptococcus bacteremia were identified by active surveillance in three hospitals. Serum samples collected within 2 days of the time of blood culture were assayed for IgG antibodies to the capsular polysaccharide of the infecting strain: serotype Ia (3 isolates), III (5 isolates), or V (4 isolates). In 7 of 12 bacteremia episodes, the serum level of IgG to the infecting isolate was > or = 3.5 microg/mL, higher than the 1-2 microg/mL level thought to be protective in neonates. Among selected acute-phase sera, 4 of 5 that contained > or = 3.5 microg/mL specific IgG mediated efficient opsonophagocytic killing of the corresponding group B streptococcus isolate in vitro. High levels of specific antibodies during the acute phase of invasive group B streptococcus infection in nonpregnant adults may reflect a rapid antibody response to infection or, in some cases, may indicate that susceptibility is due to defects in other immune effectors.