Deficient alternative complement pathway activity in newborn sera.

Neonatal susceptibility to overwhelming bacterial infection is commonly attributed to a relative deficiency in serum opsonic activity. However, few studies have compared the functional capacity of the classical complement pathway with that of the alternative complement pathway in the neonate. The opsonic activity of nine maternal infant serum pairs were studied by determining percent uptake of radiolabeled Escherichia coli. Seven mother-infant paired sera were studied using E. coli strains known to be opsonized via the alternative complement pathway: the mean percent uptake of E. coli opsonized in neonatal sera was 16.8%; of those opsonized in maternal sera, 54%; and of those opsonized in control sera, 45% (P less than 0.005). Two E. coli strains requiring the classical complement pathway for opsonization were phagocytized equally well in maternal and infant sera of seven mother-infant pairs. Determination of anti-O hemagglutination inhibition (HI) antibody titers in six maternal sera for one classical complement pathway activating and one alternative complement pathway strain showed no correlation between percent phagocytosis and HI antibody titer. These data would suggest that serum levels of classical pathway components are probably adequate for opsonization of E. coli via the classical pathway, but that low alternative complement pathway activity in neonatal sera may contribute to the newborn's increased susceptibility to bacterial sepsis.