Chitinases are digestive enzymes that break down glycosidic bonds in chitin. Because chitin composes the cell walls of fungi and exoskeletal elements of some animals (including worms and arthropods), chitinases are generally found in organisms that either need to reshape their own chitin or to dissolve and digest the chitin of fungi or animals.
Chitinivorous organisms include many bacteria (Aeromonads, Bacillus, Vibrio, among others), which may be pathogenic or detritivorous. They may attack arthropods, zooplankton or fungi; or they may degrade the remains of these organisms.
Fungi, such as Coccidioides immitis, are known to possess chitinases. This may be related to their typical role as detritovores and also to their potential as pathogens, infiltrating arthropods.
Plants may seem an unusual source for chitinase, but some of the archetypical chitinases have been characterized from plants (barley seed chitinase: PDB 1CNS, EC 184.108.40.206). Some plant chitinases are members of the pathogenesis related (PR) proteins which are induced after systemic acquired resistance induction (biotic and abiotic). Expression is mediated by the NPR1 gene and the salicylic acid pathway, both involved in resisting fungal and insect attack. Some may be required for creating fungal symbioses.
Chitin, like cellulose, has been thought of as abundant but difficult to digest. It is typically considered unavailable carbohydrate in animal diets, though certain fish can digest chitin to sugar; and dogma suggests that just as ruminants need bacteria to digest cellulose, chitin digestion would also require symbiosis and lengthy fermentations. As such, the discovery of animal, and particular mammalian and human chitinases is somewhat surprising. Actually, human chitinases appear in gastric juices . This is likely to be digestive chitinase, for catabolic activity.
Further, chitinase activity can be detected systemically in humans, in the blood  and possibly cartilage . This is related, as is plant chitinase activity, to inflammation/pathogen resistance. As such, it is unsurprisingly related to allergies. What is surprising, perhaps, is that asthma in particular has been linked to enhanced chitinase expression levels. This may begin to explain some of the most common allergies (dust mites, mold spores - both chitin covered) and speak to the relationship between allergies and worm (helminth) infections, as part of one version of the hygiene hypothesis  (worms have chitinous mouthparts to hold the intestinal wall). Finally, the link between chitinases and salicytic acid in plants is well established - but there is a hypothetical link between salicytic acid and allergies in humans (Fiengold). The link between chitinases and allergies being now established provides a tantalizing thread that can also connect these.
^ Sámi L, Pusztahelyi T, Emri T, Varecza Z, Fekete A, Grallert A, Karányi Z, Kiss L, Pócsi I (2001). "Autolysis and aging of Penicillium chrysogenum cultures under carbon starvation: Chitinase production and antifungal effect of allosamidin". The Journal of General and Applied Microbiology47 (4): 201-211.
^ Xiao X, Yin X, Lin J, Sun L, You Z, Wang P, Wang F (2005). "Chitinase Genes in Lake Sediments of Ardley Island, Antarctica". Applied and Environmental Microbiology71 (12): 7904-7909.
^ Hunt DE, Gevers D, Vahora NM, Polz MF (2008). "Conservation of the Chitin Utilization Pathway in the Vibrionaceae". Applied and Environmental Microbiology74 (1): 44-51.
^ Salzer P, Bonanomi A, Beyer K, Vögeli-Lange R, Aeschbacher RA, Lange J, Wiemken A, Kim D, Cook DR, Boller T (2000). "Differential Expression of Eight Chitinase Genes in Medicago truncatula Roots During Mycorrhiza Formation, Nodulation, and Pathogen Infection". Molecular Plant-Microbe Interactions13 (7): 763-777.
^ Akaki C, Duke GE (2005). "Apparent chitin digestibilities in the Eastern screech owl (Otus asio) and the American kestrel (Falco sparverius)". Journal of Experimental Zoology283 (4-5): 387-393.
^ Gutowska MA, Drazen JC, and Robison BH (in press). "Digestive chitinolytic activity in marine fishes of Monterey Bay, California". Comparative Biochemistry and Physiology B.
^ Paolettia MG, Norbertob L, Daminia R, Musumeci S (2007). "Human Gastric Juice Contains Chitinase That Can Degrade Chitin". Annals of Nutrition and Metabolism51 (3): 244-251.
^ Renkema GH, Boot RG, Muijsers AO, Donker-Koopman WE, Aerts JMFG (1995). "Purification and Characterization of Human Chitotriosidase, a Novel Member of the Chitinase Family of Proteins". The Journal of Biological Chemistry270 (5): 2198-2202.
^ Escott GM, Adams DJ (1995). "Chitinase activity in human serum and leukocytes". Infection and Immununity63 (12): 4770-4773.
^ Hakala BE, White C, Recklies AD (1993). "Human cartilage gp-39, a major secretory product of articular chondrocytes and synovial cells, is a mammalian member of a chitinase protein family". The Journal of Biological Chemistry268 (34): 25803-25810.
^ Recklies AD, White C, Ling H (2002). "The chitinase 3-like protein human cartilage glycoprotein 39 (HC-gp39) stimulates proliferation of human connective-tissue cells and activates both extracellular signal-regulated kinase- and protein kinase B-mediated signalling pathways". Biochemistry Journal365 (1): 119–126.
^ van Eijk M, van Roomen CPAA, Renkema GH, Bussink AP, Andrews L, Blommaart EFC, Sugar A, Verhoeven AJ, Boot RG, Aerts JMFG (2005). "Characterization of human phagocyte-derived chitotriosidase, a component of innate immunity". International Immunology17 (11): 1505-1512.
^ Bierbaum S, Nickel R, Koch A, Lau S, Deichmann KA, Wahn U, Andrea Superti-Furga A, Heinzmann A (2005). "Polymorphisms and Haplotypes of Acid Mammalian Chitinase Are Associated with Bronchial Asthma". American Journal of Respiratory and Critical Care Medicine172: 1505-1509.
^ Zhao J, Zhu H, Wong CH, Leung KY, Wong WSF (2005). "Increased lungkine and chitinase levels in allergic airway inflammation: A proteomics approach". Proteomics5 (11): 2799 - 2807.
^ Elias J, Homer R, Hamid Q, Lee C (2005). "Chitinases and chitinase-like proteins in TH2 inflammation and asthma". Journal of Allergy and Clinical Immunology116 (3): 497-500.
^ Zhu Z, Zheng T, Homer RJ, Kim YK, Chen NY, Cohn L, Hamid Q, Elias JA (2004). "Acidic Mammalian Chitinase in Asthmatic Th2 Inflammation and IL-13 Pathway Activation". Science304 (5677): 1678-1682.
^ Chupp GL, Lee CG, Jarjour N, Shim YM, Holm CT, He S, Dziura JD, Reed J, Coyle AJ, Kiener P, Cullen M, Grandsaigne M, Dombret MC, Aubier M, Pretolani M, Elias JA (2005). "A Chitinase-like Protein in the Lung and Circulation of Patients with Severe Asthma". The New England Journal of Medicine357 (20): 2016-2027.
^ Maizels RM (2005). "Infections and allergy — helminths, hygiene and host immune regulation". Current Opinion in Immunology17 (6): 656-661.
^ Hunter MM, Mckay DM (2004). "Helminths as therapeutic agents for inflammatory bowel disease". Alimentary Pharmacology & Therapeutics19 (2): 167–177.
^ Palmas C, Gabriele F, Conchedda M, Bortoletti G, Ecca AR (2003). "Causality or coincidence: may the slow disappearance of helminths be responsible for the imbalances in immune control mechanisms?". Alimentary Pharmacology & Therapeutics77 (2): 147-153.