An enzyme that catalyses the hydrolysis of chitin, a major constituent of fungal cell walls and of the exoskeleton of insects and other arthropods. It is synthesized by certain insect-eating animals, such as frogs, and also by certain plants as part of their defence against fungal infection (see hypersensitivity). Transfer of the chitinase gene between different plant species has been successfully undertaken in an attempt to enhance the resistance of susceptible species to pathogenic fungi. Human macrophages can also secrete a chitinase enzyme, which may help to destroy chitin-containing pathogens, and chitinase-like proteins are expressed in several diseases, including asthma, cystic fibrosis, and other inflammatory conditions.