Passive protection by human sera in mice against challenge with strains of group B streptococci

The passive protective activity of 100 human sera in mice challenged with strains of group B streptococci (GBS), types Ia, Ib, Ic, II and III, was investigated. The number of sera which offered protection against type strains III, II and Ib were 10, 6 and 1, respectively, while 5 sera gave protection against type strains Ia and Ic. The activity was 2-mercaptoethanol sensitive while anti-human IgG, IgA and IgM rabbit sera were capable of negating the protective activity. Furthermore, type antigens removed the protective activity, resulting in the reduction of the amounts of IgG, IgA and IgM in normal human serum. These findings suggest that the protective activity of human sera is related to specific immunoglobulins against type antigens of GBS.