Pulmonary collectins orchestrate host defense, inflammation, and oxidant production. Schematic of alveolar defense roles for SP-A and SP-D suggested by in vitro studies and genetically engineered animal models. SP-A and SP-D are secreted by alveolar type II cells into the alveolar lumen. The presentation of microorganisms to inflammatory cells is determined in part by SP-A and SP-D, which aggregate and opsonize diverse viral, fungal, and bacterial species. SP-A and SP-D preserve gas exchange by limiting inflammatory cell infiltration and alveolar exudation, both by increasing the rate of clearance of microorganisms from the lung and by directly modulating inflammatory cytokine and oxidant responses.