Prostanoids are a series of arachidonic acid metabolites produced by the action of cyclooxygenase and subsequently by isomerases and synthases. Cells rapidly secrete prostanoids after synthesis, whereupon the prostanoids bind to a family of 8 GPCRs to exert their biological effects. The prostaglandin PGD2 is produced by mast cells upon activation by allergens, and is present at high levels in allergic diseases. PGD2 binds to two receptors, DP and CRTH2. DP activates Gs to increase cAMP levels, and lack of DP results in reduced allergic response in animal models of bronchial asthma.