Arachidonic acid plays a central role in a biological control system where such oxygenated derivatives as prostaglandins, thromboxanes, and leukotrienes are mediators. The leukotrienes are formed by transformation of arachidonic acid into an unstable epoxide intermediate, leukotriene A₄, which can be converted enzymatically by hydration to leukotriene B₄, and by addition of glutathione to leukotriene C₄. This last compound is metabolized to leukotrienes D₄ and E₄ by successive elimination of a γ-glutamyl residue and glycine. Slow-reacting substance of anaphylaxis consists of leukotrienes C₄, D₄, and E₄. The cysteinyl-containing leukotrienes are potent bronchoconstrictors, increase vascular permeability in postcapillary venules, and stimulate mucus secretion. Leukotriene B₄ causes adhesion and chemotactic movement of leukocytes and stimulates aggregation, enzyme release, and generation of superoxide in neutrophils. Leukotrienes C₄, D₄, and E₄, which are released from the lung tissue of asthmatic subjects exposed to specific allergens, seem to play a pathophysiological role in immediate hypersensitivity reactions. These leukotrienes, as well as leukotriene B₄, have pro-inflammatory effects.