Quercetin

Quercetin is a type of flavonoid (plant phytonutrients) that can be found in fruits and vegetables, and is one of the most studied flavonoids for its potential health benefits. 

Quercetin is a powerful antioxidant that helps to protect body cells from free radicals damage, thus protecting us from premature aging and degenerative diseases such as heart disease, diabetes, arthritis, and etc. It also posses anti-inflammatory property which help to alleviate inflammation, thus may be helpful in maintaining heart health and managing joint pain associated with arthritis.

It was suggested that quercetin might be helpful to relieve allergy symptoms by preventing the release of histamine (a chemical that is involved in allergic symptoms such as itching and sneezing) from immune cells. Being a powerful antioxidant, quercetin might have anti-cancer properties that help to suppress tumour growth and prevent the spread of cancer cells. 
Common food sources are citrus fruits, grapes, berries, cherries, buckwheat, broccoli, kale, onions, and red wines.