Type-specific antibody prevents platelet aggregation induced by group B streptococci type III.

Group B streptococci (GBS) type III organisms readily induced platelet aggregation and serotonin release in human platelet-rich plasma (PRP). In a system using a GBS/platelet ratio of 1.5, aggregation occurred after 2 to 9 minutes (maximum aggregation, 73% +/- 11%). Serotonin release began within the first minute, reaching 40% before aggregation was detected. Maximum release was 65% +/- 9%. The addition of type-specific rabbit antisera inhibited aggregation and release in a dose-dependent fashion, whereas rabbit antisera against GBS type II and a pneumococcus type 14 and 19 had no effect. To test the activity of different isotypes, monoclonal antibodies against the sialic acid determinant of the GBS type III antigens were used. IgG, IgM, and IgA antibodies were all effective in blocking aggregation and serotonin release. Although the significance of this phenomenon is not clear, it may represent a protective function of antibody that is not directly related to opsonization and phagocytosis.