Defective activation of the third component of complement in the sera of newborn infants

The activation of the terminal complement components, C3-9, plays an important role in the host’s defense against infection. In the present study, the ability of bacteria to activate the third component of complement (C3) in newborn serum was examined. A variety of bacteria were incubated in test sera at 37°C for 30 min and the percent of available C3 that was activated was measured. Using one strain of Escherchia coli (#3), 32% (mean) of the available C3 was activated in sera from 18 newborns, as compared to 85% in sera from their mothers and 79% in sera from 13 normal adults (P < 0.005). In contrast, using another strain of E. coli (N70), the percent of C3 activated in newborn sera (83%) was the same as in sera from their mothers (81%) or in sera from normal adults (73%). The defective activation of C3 in newborn sera by E. coli was not related to the presence of the K1 antigen. Newborn sera were also challenged with other bacterial species and the activation of C3 was deficient when tested with klebsiellae, but not with staphylococci or streptococci. The defect in newborn sera appeared to be due to a deficiency of a serum factor rather than to the presence of an inhibitor. Speculation: The defective activation of C3-9 in newborn sera by some bacteria may be, in part, responsible for the neonate’s increased susceptibility to infection.