Leukotrienes are products of the enzyme arachidonate 5-lipoxygenase (5-LO). They are fatty molecules that contribute to inflammation in asthma and allergic rhinitis. Leukotrienes are typically biosynthesized in concert with histamine.  As the name suggests, arachidonic acid (AA) is a substrate of arachidonate 5-lipoxygenase.  AA is an omega-6 fatty acid.  Eicosapentaenoic acid (EPA) is also a 5-LO substrate.  EPA is an omega-3 fatty acid.  The 4-series leukotrienes (LTB4, LTC4, LTD4, LTE4) are synthesized from AA. They are generally proinflammatory. The 5-series leukotrienes (LTB5, LTC5, LTD5, LTE5) are synthesized from EPA. They are anti-inflammatory.  Leukotrienes are released from mast cells as part of the inflammation response. They attract white blood cells to sites of tissue damage and cause smooth muscles to contract.