Quercetin, a flavonoid found in fruits and vegetables, has various biological activities including anti-inflammatory, antioxidant, and immunomodulatory effects. This review summarizes the physicochemical properties, dietary sources, absorption, bioavailability, and metabolism of quercetin, with a focus on its effects on inflammation and immune function. Quercetin is a polyphenol that is absorbed in the small intestine and metabolized in the liver and kidneys. It is found in a variety of foods, including apples, berries, onions, tea, and tomatoes, and its intake varies by country. Quercetin is absorbed in the upper segment of the small intestine, where it is methylated, sulfo-substituted, and glucuronidated. It is excreted primarily through the kidneys. In vitro studies show that quercetin has anti-inflammatory and immunomodulatory effects, inhibiting the production of inflammatory mediators and modulating immune cell function. In vivo studies in animal models also support these findings, showing that quercetin can reduce inflammation and improve immune function. However, clinical studies in humans have shown mixed results, with some studies indicating that quercetin may reduce the risk of upper respiratory tract infections and improve immune function, while others show no significant effects. Overall, quercetin has potential as a dietary supplement with anti-inflammatory and immunomodulatory properties, but further research is needed to fully understand its mechanisms and effects in humans.Quercetin, a flavonoid found in fruits and vegetables, has various biological activities including anti-inflammatory, antioxidant, and immunomodulatory effects. This review summarizes the physicochemical properties, dietary sources, absorption, bioavailability, and metabolism of quercetin, with a focus on its effects on inflammation and immune function. Quercetin is a polyphenol that is absorbed in the small intestine and metabolized in the liver and kidneys. It is found in a variety of foods, including apples, berries, onions, tea, and tomatoes, and its intake varies by country. Quercetin is absorbed in the upper segment of the small intestine, where it is methylated, sulfo-substituted, and glucuronidated. It is excreted primarily through the kidneys. In vitro studies show that quercetin has anti-inflammatory and immunomodulatory effects, inhibiting the production of inflammatory mediators and modulating immune cell function. In vivo studies in animal models also support these findings, showing that quercetin can reduce inflammation and improve immune function. However, clinical studies in humans have shown mixed results, with some studies indicating that quercetin may reduce the risk of upper respiratory tract infections and improve immune function, while others show no significant effects. Overall, quercetin has potential as a dietary supplement with anti-inflammatory and immunomodulatory properties, but further research is needed to fully understand its mechanisms and effects in humans.