The leukotrienes are a family of lipid mediators formed by the action of the 5-lipoxygenase enzyme (5-LO) and 5-lipoxygenase-activating protein (FLAP) on the cell membrane phospholipid, arachidonic acid. The functionally active members of the leukotriene family are leukotriene B4 and leukotrienes C4, D4 and E4. The last three in this list are collectively known as the cysteinyl leukotrienes. Leukotrienes are potent biological mediators of inflammation and have been implicated in the pathogenesis of airway inflammatory diseases such as asthma and allergic rhinitis. They were first identified in 1976 when Samuelson et al showed that lipoxygenase enzymes in rabbit polymorphonuclear monocytes could oxygenate arachidonic acid at C-5. This produced monohydroxy acids that would later become known as the leukotrienes.