Lipid-binding protein which shows high specificity for the surfactant phospholipid dipalmitoylphosphatidylcholine (DPPC). Plays a role in the innate immune responses of the upper airways. Reduces the surface tension in secretions from airway epithelia and inhibits the formation of biofilm by pathogenic Gram-negative bacteria, such as P.aeruginosa and K.pneumoniae. Negatively regulates proteolytic cleavage of SCNN1G, an event that is required for activation of the epithelial sodium channel (ENaC), and thereby contributes to airway surface liquid homeostasis and proper clearance of mucus. Plays a role in the airway inflammatory response after exposure to irritants. May attract macrophages and neutrophils. Monomer. Interacts (via N-terminus) with SCNN1B, a subunit of the heterotrimeric epithelial sodium channel (ENaC); this inhibits proteolytic activation of ENaC (By similarity). Apical side of airway epithelial cells. Detected in airway surface liquid, nasal mucus and sputum (By similarity). Expressed in trachea, and at lower levels in nasal epithelium. Belongs to the BPI/LBP/Plunc superfamily. Plunc family. Reported to bind to bacterial lipopolysaccharide (LPS) in vitro. However, the in vivo significance of this is uncertain since other studies indicate little or no specificity for LPS.