Complement in the Newborn Infant

Abstract

Classical pathway activities and component concentrations in sera from newborns can be compared more realistically to normal adult values than to maternal values since activities and components are increased in maternal sera. Whole complement hemolytic activity appears to be subnormal in approximately half of term infants, with mean activity being about 70% to 90% of normal. Concentrations of Clq, C4, C2, C3, and C7 have been 60% to 100% of adult concentrations in term infants and somewhat less in preterm infants. Younger gestational age has been correlated with lower levels of total hemolytic activity, Clq, C4, and C3. Activity of the alternative pathway appears to be more frequently subnormal in newborn sera than does activity of the classical pathway. Factor B and properdin concentrations have varied from about 35% to 60% and 35% to 70% of adult values, respectively. Opsonization and hemolysis mediated by the alternative pathway have been subnormal in 15% to 75% of term infants, depending upon the assay. Gestational age appears to correlate with alternative pathway hernobytic activity and properdin concentration but not with concentration of factor B. Reductions such as these in single complement components and functions probably would not predispose an otherwise normal individual to infection. However, it seems likely that the multicomponent and dual pathway deficiencies found in neonates, especially in conjunction with the decreased phagocyte function known to exist in that population, could increase the likelihood of serious infection. Predicted infections with this configuration of abnormalities would be extracellular bacterial. Whether the newborn infant is actually predisposed to infection because of the complement deficiencies summarized here remains to be proved. This and other unanswered questions lead us to conclude that understanding of the complement system in the newborn is, pardon the expression, still in its infancy.