Usually used in conjunction with quercetin as an alternative therapy for allergies is bromelain, a protein-digesting enzyme found in pineapples. Bromelain is typically marketed as a natural anti-inflammatory for the treatment of arthritis, sinusitis and injuries where there is swelling and inflammation, and in allergies, where it boosts the effect of quercetin in suppressing the inflammation of allergic reactions.

Through its action on the blood clotting-related substances fibrinogen and fibrin, bromelain stimulates the production and release of anti-inflammatory prostaglandins (PGs), while simultaneously reducing the production and release of proinflammatory PGs.

Allergic reactions typically involve an excess of inflammatory PGs being released, which contributes to the swelling, redness and itching. [Quercetin, QRC] also helps to suppress formation and release of inflammatory PGs and thromboxanes, as well as the "slow reacting substance of anaphylaxis," a key trigger of asthma.

Thus, QRC and bromelain are synergistic in suppressing the inflammation of allergic reactions, as well as the excessive inflammation that results from bruising and tissue damage from sports injuries, accidents, surgery, etc.

Bromelain also potentiates QRC through another route. By itself, QRC is somewhat poorly absorbed from the GI tract. Bromelain is well known for its enhancement of absorption of various compounds, including pentobarbital and antibiotics such as tetracycline and amoxicillin. Bromelain also enhances the absorption of QRC.

For a list of the scientific literature regarding quercetin and bromelain, refer to this article.