Das Flavonoid Quercetin und das Immunsystem: Foods rich in quercetin include black and green tea apples (440 mg/kg), onion, especially red onion (191 mg/kg) (higher concentrations of quercetin occur in the outermost rings and in the part closest to the root, the latter being the part of the plant with the highest concentration), red grapes, citrus fruit, tomato, broccoli and other leafy green vegetables, and a number of berries (Bilberries) http://de.wikipedia.org/wiki/Quercetin ★ Quercetin glycosides occur in broccoli, apples, and especially in onions, with an abundance as high as 0.25–0.5 g/kg