Quercetin is a natural flavonoid found in many fruits, vegetables, and grains. It is known for its antioxidant properties, which help protect cells from damage caused by free radicals. Common sources of quercetin include apples, onions, berries, and green tea. Research suggests that quercetin may have anti-inflammatory effects and could support immune function. Some studies indicate it may help reduce allergy symptoms and improve heart health, although more research is needed to fully understand its benefits and potential uses in health and nutrition.