To perform its function as the organ for gas exchange, the lung of the adult human interfaces directly with its environment through a surface of approximately 150 m2. This extensive surface area is necessary for gas exchanges but encompasses two basic functional demands. The gas exchange surface has to be kept available during breathing and defended against infectious agents and irritants that may be present in air or blood. Pulmonary surfactant, a highly specialized system found in all mammalian lungs, may do both. It helps to keep the gas exchange surface available during breathing, and may represent early lung defence strategies of the host avoiding air or blood-borne pulmonary infection and lung tissue injury.

The importance of a functioning surfactant system to lung function was first demonstrated in a clinical setting by the acute respiratory distress syndrome (RDS) of the premature newborn. The surfactant system of these infants is not fully developed [l], and some of them respond dramatically to intratracheally administered exogenous surfactants [2, 3]. In addition, surfactant abnormalities have been demonstrated for other respiratory diseases than RDS in humans and animals. These diseases include the adult respiratory distress syndrome (ARDS)[4-71, AIDS associated