Complement function and the synthesis of lung surfactant may be a regulation which preterm infants have in common.

The levels of CH50, the complement components, C1q, C4, and C3, C3 degradation fragments, and factor B were determined in the cord blood of 128 newborn infants. The levels of C3, C4, and C3d3 (an index of chronic in vivo complement activation) were clearly lower in the 28 infants with respiratory distress syndrome (RDS) than in the infants with other lung diseases or with normal lungs. CH50 and factor B levels were also low in RDS. Levels of C1q and other serum components in RDS infants were similar to the average levels in other infants without RDS at a corresponding gestational age. Lung surfactant is synthesized in alveolar type 2 cells, in which the complement components C4, C3, and factor B, but not C1q, have been reported to be synthesized. It seems possible that common factors regulate the synthesis of some complement components and surfactant.