Degrades chitin and chitotriose. May participate in the defense against nematodes, fungi and other pathogens. Plays a role in T-helper cell type 2 (Th2) immune response. Contributes to the response to IL-13 and inflammation in response to IL-13. Stimulates chemokine production by pulmonary epithelial cells. Protects lung epithelial cells against apoptosis and promotes phosphorylation of AKT1. Its function in the inflammatory response and in protecting cells against apoptosis is inhibited by allosamidin, suggesting that the function of this protein depends on carbohydrate binding. Presence in saliva and gastric juice suggests a function as a digestive enzyme. Random endo-hydrolysis of N-acetyl-beta-D-glucosaminide (1->4)-beta-linkages in chitin and chitodextrins. Optimum pH is below 2.5. Interacts with EGFR. Detected in secretory granules of parotid acinar cells and gastric chief cells and secreted from them into saliva and gastric juice, respectively. Detected in macrophages and lung epithelial cells. Detected in the acinar cells of parotid gland and von Ebner's gland but not in submandibular and sublingual glands. Detected in gastric chief cells. Also present in parotid glandular saliva and gastric juice (at protein level). Highly expressed in submandibular gland (PubMed:11085997) and stomach. Highly expressed in parotid gland but not in submandibular and sublingual glands (PubMed:12133911). In tongue, expressed only in von Ebner's gland. Expressed at lower levels in lung. In parotid gland, weak expression detected at postnatal day P12, with levels increasing towards P16. In stomach, first detected at P16, with expression reaching adult levels during P20-24. Up-regulated upon pulmonary inflammation elicited by sensitization and aerosol challenge with the aeroallergen ovalbumin. Up-regulated during T-helper cell type 2 (Th2) inflammation. Induction is mediated by IL-13. Belongs to the glycosyl hydrolase 18 family. Chitinase class II subfamily. Truncated N-terminus.