Quercetin is a bioflavonoid found in onions, apples, black tea, and grapefruit. Quercetin has antioxidant activity and is used in herbal medicine as a blood vessel protectant. Mast cells contribute to the inflammatory process in allergic reactions, where immunologic stimulation leads to degranulation of the mast cell, as well as to the generation of numerous cytokines and inflammatory mediators. Many studies have explored the effect of quercetin for managing allergies, as it has been shown to inhibit mast-cell secretions and has the ability to downregulate histamine production via inhibition of the enzyme histidine decarboxylase (HDC) mRNA from human mast cells.