Quercetin is a powerful antioxidant found in red grapes, red and yellow onions, broccoli, and other foods.

In addition to quercetin's ability to fight free radicals, it also fights inflammation and histamine (or allergic) reactions. 

Recent research suggests that quercetin may help to fight cancers of the prostate, colon and skin by causing cancer cells to self-destruct.

Because quercetin is highly concentrated in the skins of fruits and veggies, make sure they're organic, and don't peel for maximum potency.

References

Jung WJ, Sung MK.Effects of major dietary antioxidants on inflammatory markers of RAW 264.7 macrophages.Biofactors. 2004;21(1-4):113-7. Caltagirone S, Rossi C, Poggi A, Ranelletti FO, Natali PG, Brunetti M, Aiello FB, Piantelli M Flavonoids apigenin and quercetin inhibit melanoma growth and metastatic potential. Int J Cancer 2000 Aug 15;87(4):595-600 Vijayababu MR, Kanagaraj PP, Arunkumar AA, Arunakaran JJ.Effects of quercetin on insulin-like growth factors (IGFs) and their binding protein-3 (IGFBP-3) secretion and induction of apoptosis in human prostate cancer cells.J Carcinog. 2006 Apr 6;5(1):10.