Protective enzyme has a downside: Asthma.

The article examines how an enzyme whose natural job may be to ward off fungi and parasites contributes to the lung inflammation characteristic of asthma. The enzyme is known as a chitinase because it breaks down the complex sugar chitin, a tough molecule found in the cell walls of fungi, the surface of parasitic worms, and the exoskeletons of insects and crustaceans. Mammals don't make chitin, but people have many genes encoding chitinases, so biologists consider the enzymes to be part of the human immune response. Jack A. Elias of Yale University Medical School in New Haven and his colleagues have now found that mice given a substance that triggers asthmalike lung inflammation produce large amounts of a chitinase in their lungs. Blocking the enzyme with an antibody or another compound decreases the inflammation. Elias' team further discovered that lung-tissue samples from asthmatic people, but not samples from other people, contain significant amounts of the chitinase. Some researchers have suggested that asthma is on the rise because people living in developed countries now face fewer parasitic infections than people did in the past.