A class of compounds synthesized from arachidonic acid in a variety of tissues, including the lungs and leucocytes. There are at least six different types of leukotrienes, all of which play an active role in regulating inflammation. In the immune response, LTs attract neutrophils and eosinophils to sites of infection, increase the permeability of small blood vessels, and cause vasoconstriction. They play a key role in various inflammatory and allergic diseases such as asthma.