Antioxidant Bioflavonoid for Normal Histamine Function* A Dietary Supplement Quercetin is an antioxidant bioflavonoid found in such foods as broccoli, grapes and red and yellow onions.* It has been found to block destructive structural changes in cells, thus helping to maintain normal cell growth.* Quercetin also inhibits the synthesis of enzymes which help prevent the release of IgG-mediated histamine from mast cells and basophils.*