Quercetin and Its Lecithin-Based Formulation: Potential Applications for Allergic Diseases Based on a Narrative Review.

Quercetin, a natural flavonoid, present in various vegetables and fruits, has garnered increasing attraction for its potential antiallergic properties. Its broad-spectrum activity depends on its anti-inflammatory, immunomodulatory, and antioxidant effects, which target the critical pathways involved in type 2-driven allergic inflammation. Quercetin inhibits mast cell degranulation, reduces the production of histamine and pro-inflammatory cytokines, and restores homeostasis of the immune system by modulating the Th1/Th2 and Treg/Th17 balances. Additionally, its antioxidant properties help to dampen oxidative stress, a critical factor in the pathophysiology of allergic diseases. In vitro studies have consistently demonstrated quercetin's ability to suppress allergic reactions. In contrast, in vivo studies, particularly in murine models of allergic rhinitis, have confirmed its efficacy in relieving symptoms (such as nasal itching, sneezing, rhinorrhea, and congestion) and dampening type 2 mucosal inflammation. Preclinical evidence also supports its therapeutic potential in asthma, conjunctivitis, atopic dermatitis, and food allergies. However, human studies are still scarce, as only two clinical trials investigated quercetin as a monotherapy. Both studies reported promising results, including symptom reduction and improved quality of life, though larger, randomized trials are needed to validate these findings. Some other studies have investigated multicomponent products that also contain quercetin. This review aimed to report and discuss the most recent in vitro and in vivo evidence on quercetin's application in allergic models. It also provides a comprehensive overview of human studies, highlighting its potential as an agent in food supplements to manage patients with allergic diseases. Moreover, this review introduces a new quercetin phospholipids formulation that may represent a keystone in clinical use. The literature search was based on a PubMed consultation considering the most recent (last five years) publications using the keywords "quercetin and allergic disease" and "quercetin and immune system".