Onions are a prebiotic, meaning they are needed to fuel the “good” bacteria in the gut. It is important to keep the digestive tract happy as it houses seventy-five percent of the immune system and controls how the body reacts to allergens. Quercetin is the active ingredient found in onions. It is responsible for fighting inflammation, boosting the immune system, and inhibiting the release of histamines, or chemical substances that induce allergic reactions. Other good sources of quercetin include broccoli, berries, and apples.