Gel-forming glycoprotein of gastric and respiratory tract epithelia that protects the mucosa from infection and chemical damage by binding to inhaled microorganisms and particles that are subsequently removed by the mucociliary system (PubMed:14535999, PubMed:14718370). Interacts with H.pylori in the gastric epithelium, Barrett's esophagus as well as in gastric metaplasia of the duodenum (GMD) (PubMed:14535999). Multimeric. Highly expressed in surface mucosal cells of respiratory tract and stomach epithelia. Overexpressed in a number of carcinomas. Also expressed in Barrett's esophagus epithelium and in the proximal duodenum. In airway epithelial cells, expression increases significantly during cell differentiation (at protein level). By IL4. The cysteine residues in the Cys-rich subdomain repeats are not involved in disulfide bonding. C-, O- and N-glycosylated (PubMed:14718370). O-glycosylated on the second and last Thr of the Thr-/Ser-rich tandem repeats TTPSPVPTTSTTSA (PubMed:25939779, PubMed:14718370, PubMed:22186971). One form of glycosylation is also known as Lewis B (LeB) blood group antigen, a tetrasaccharide consisting of N-acetylglucosamine having a fucosyl residue attached (PubMed:14535999). It has a role as an epitope and antigen and functions as a receptor for H.pylori binding and facilitates infection (PubMed:14535999). C-mannosylation in the Cys-rich subdomains may be required for proper folding of these regions and for export from the endoplasmic reticulum during biosynthesis (PubMed:14718370). Proteolytic cleavage in the C-terminal is initiated early in the secretory pathway and does not involve a serine protease. The extent of cleavage is increased in the acidic parts of the secretory pathway. Cleavage generates a reactive group which could link the protein to a primary amide. Truncated C-terminus.