Perinatal immunity to group B beta-hemolytic streptococcus type Ia.

Sera from 14 of 56 adult women protected mice from intraperitoneal challenge with mouse-virulent group B Streptococcus serotype Ia, and sera from seven of 25 nonparturient women in this group were bactericidal for greater than 99% of the organisms in the presence of normal polymorphonuclear leukocytes. There were no discrepancies between the in vivo and in vitro assays. Protective activity was found in the IgG class in seven sera, and in the IgM class in one. Opsonic activity was partially dependent on heat-labile serum factors. Of 31 mother-cord serum pairs studied, seven maternal sera were protective, but four of the corresponding cord sera were not. Pooled human gamma-globulin injected by either the intraperitoneal or the intramuscular route protected mice from bacterial challenge.