Quercetin is a yellow-colored bioflavanoid found primarily in apples and onions. Quercetin has research suggesting it acts as an antiviral, anti-cancer, and an anti-inflammatory, and that it has shown benefits in patients with Fibromyalgia and Metabolic Syndrome. Quercetin has a primary effect of stabilizing mast cells, which are the cells that release histamine in the body during an allergic reaction. It has been used historically to both treat and prevent allergies (environmental, as well as food).