Lipid-binding protein which shows high specificity for the surfactant phospholipid dipalmitoylphosphatidylcholine (DPPC) (PubMed:25223608). Plays a role in the innate immune responses of the upper airways (PubMed:23499554, PubMed:23132494). 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 (PubMed:23499554, PubMed:23132494, PubMed:27145151). 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 (PubMed:24124190, PubMed:24043776). Plays a role in the airway inflammatory response after exposure to irritants (PubMed:11425234). May attract macrophages and neutrophils (PubMed:23132494). Monomer. Interacts (via N-terminus) with SCNN1B, a subunit of the heterotrimeric epithelial sodium channel (ENaC); this inhibits proteolytic activation of ENaC. Apical side of airway epithelial cells. Detected in airway surface liquid, nasal mucus and sputum. Highly expressed in lung, upper airways and nasopharyngeal regions, including trachea and nasal epithelium (at protein level) (PubMed:11018263, PubMed:11251963, PubMed:12409287, PubMed:11425234, PubMed:26559477). Specifically expressed in the secretory ducts and submucosal glands of tracheobronchial tissues (at protein level) (PubMed:12409287, PubMed:11425234). Also expressed in the eye where it is detected in lacrimal gland, eyelid, conjunctiva and cornea (at protein level) (PubMed:26559477). Specifically localizes to epithelial cell layers in cornea, eyelid (basal epithelium) and conjunctiva (at protein level) (PubMed:26559477). Detected within acinar cells and ducts in the lacrimal and Meibomian glands (at protein level) (PubMed:26559477). In lung, shows highest expression in the trachea and progressive decrease from proximal (bronchial) to distal (bronchiolar) airways (PubMed:12409287). Also expressed in lung cancers and some other types of cancer (PubMed:11251963). Up-regulated in response to all-trans retinoic acid (ATRA) (PubMed:12409287). Up-regulated in tear fluid of patients suffering from dry eye disease (PubMed:26559477). Up-regulated in response to the pro-inflammatory cytokines IL1B and TNF, and the bacteria E.coli and P.aeruginosa (PubMed:26559477). May be N-glycosylated. Belongs to the BPI/LBP/Plunc superfamily. Plunc family. Reported to bind to bacterial lipopolysaccharide (LPS) in vitro (PubMed:15158712, PubMed:27145151). However, the in vivo significance of this is uncertain since other studies indicate little or no specificity for LPS (PubMed:12920053, PubMed:25223608).