Eicosanoids are 20-carbon compounds derived primarily from the fatty acid arachidonic acid. There are three medically important classes of eicosanoids: Prostanoids (prostaglandins and thromboxane) Prostanoids are mediators of pain, inflammation, fever, blood clotting, smooth muscle contraction and dilation, and protection of the gastrointestinal tract. Leukotrienes Leukotrienes participate in early stages of the immune response. They are also powerful bronchioconstrictors. Lipoxins, Resolvins and Protectins These compounds participate in resolving the immune response.