Stabilising mast cells and the histamine link
Quercetin is a flavonoid found in onions, apples and capers and is often marketed for seasonal allergy support. Its best-known mechanism is mast-cell stabilisation: in laboratory studies it reduces the degranulation step in which mast cells release stored histamine and other mediators that drive sneezing, itch and congestion. By dampening that release rather than blocking the histamine receptor, it works upstream of a standard antihistamine.
Antioxidant activity and iron chelation
Quercetin also acts as an antioxidant and can chelate (bind) iron, which contributes to its free-radical-scavenging behaviour and is part of why it is studied in oxidative-stress contexts. A persistent limitation is bioavailability: plain quercetin is poorly absorbed, so products often pair it with enzymatically modified forms or with absorption enhancers.
Human allergy trials are limited and effect sizes modest, so quercetin should be seen as a possible adjunct rather than a replacement for proven allergy treatment. Its iron-binding may matter for people managing iron status. This is general educational information, not medical advice.