Potential role of surfactant proteins A and D in innate lung defense against pathogens.

The physiological role of pulmonary surfactant is probably not limited to conferring mechanical stability to the alveoli. Increasing evidence suggests that surfactant components, in particular the hydrophilic surfactant proteins SP-A and SP-D, play potentially important roles in host defense mechanisms. Both SP-A and SP-D are collagenous C-type lectins (collectins) that are structurally and, perhaps, functionally related to collectins in the circulation. As will be discussed in this review, evidence is accumulating that the alveolar collectins SP-A and SP-D could be important components of a first-line defense system against infiltrating pathogenic micro-organisms and viruses.