Opsonic activity of cord blood sera against various species of microorganism.

The results of the present study on the opsonic activity of cord blood serum against various microorganisms (Staphylococcus aureus, Escherichia coli, and group B streptococci) show that the activity of cord blood serum in promoting IgG-mediated ingestion is equal to that of sera of healthy adults. This implies that IgG concentrations, as assessed by immunochemical methods, in cord blood and adult sera represent functionally similar IgG activities. Ingestion of microorganisms involving complement-dependent opsonization was found to be of the same level for cord blood and adult sera, when the opsonization occurred via the classical pathway of complement activation. However, due to decreased concentrations of factors B, P, and D in cord blood serum, optimal opsonization of microorganisms requiring the alternative pathway of complement was impaired. Taken together, these results indicate that an opsonic defect of cord blood serum affects mainly microorganisms requiring opsonization via the alternative pathway of complement.