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Histamine receptors exhibit constitutive activity, so antihistamines can function as either a neutral receptor antagonist or an inverse agonist at histamine receptor.

Histamine produces increased vascular permeability, causing fluid to escape from capillaries into tissues, which leads to the classic symptoms of an allergic reaction — a runny nose and watery eyes. Antihistamines suppress the histamine-induced wheal response (swelling) and flare response (vasodilation) by blocking the binding of histamine to its receptors or reducing histamine receptor activity on nerves, vascular smooth muscle, glandular cells, endothelium, and mast cells.

The goal of pharmacological therapy is to prevent motion sickness, or relieve the symptoms of motion sickness, such as nausea.

Scopolamine, an anticholinergic, is used for the prevention of motion sickness and for acute treatment.

Once released, the histamine can react with local or widespread tissues through histamine receptors.

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