This review focuses on the biological properties, dietary sources, absorption, bioavailability, and metabolism of quercetin, a polyphenol derived from plants. Quercetin has been shown to have anti-carcinogenic, anti-inflammatory, and antiviral activities, as well as to attenuate lipid peroxidation, platelet aggregation, and capillary permeability. The review highlights the main effects of quercetin on inflammation and immune function, both in vitro and in vivo. In vitro studies demonstrate that quercetin inhibits the production of inflammatory cytokines and mediators, and modulates the activity of immune cells. In vivo studies in animal models support the anti-inflammatory and immunomodulatory effects of quercetin, showing reduced inflammation and improved immune function. However, clinical studies in humans have shown mixed results, with some studies indicating reduced illness rates and others finding no significant impact on immune function. The review concludes that while quercetin exhibits promising anti-inflammatory and immunomodulatory properties, further research is needed to fully understand its mechanisms of action and optimal dosing for therapeutic use.This review focuses on the biological properties, dietary sources, absorption, bioavailability, and metabolism of quercetin, a polyphenol derived from plants. Quercetin has been shown to have anti-carcinogenic, anti-inflammatory, and antiviral activities, as well as to attenuate lipid peroxidation, platelet aggregation, and capillary permeability. The review highlights the main effects of quercetin on inflammation and immune function, both in vitro and in vivo. In vitro studies demonstrate that quercetin inhibits the production of inflammatory cytokines and mediators, and modulates the activity of immune cells. In vivo studies in animal models support the anti-inflammatory and immunomodulatory effects of quercetin, showing reduced inflammation and improved immune function. However, clinical studies in humans have shown mixed results, with some studies indicating reduced illness rates and others finding no significant impact on immune function. The review concludes that while quercetin exhibits promising anti-inflammatory and immunomodulatory properties, further research is needed to fully understand its mechanisms of action and optimal dosing for therapeutic use.