The complement system of the newborn infant.

Whole complement activity (CH50), and levels of some components of the classical (C1q, C4, C3) and alternative (factor B and properdin) pathways were determined in 55 non-infected and 11 infected newborn infants. Normal newborn infants were lower than adults in all complement measurements; preterm infants being significantly lower than term infants. Complement was not effected by intrauterine growth retardation. Absence of C3 split products indicated that the deficiencies were developmental and not due to activation of the complement system. Complement levels were lower in infected infants and this was due to activation of the complement system as C3 split products were present in 54%. Because of the high incidence of split products in infected infants, incorporating a test to determine their presence may be of benefit in the diagnosis of the presence of infection in newborn infants.