When a sensitive person inhales an allergen (allergy-causing substance) like ragweed pollen, the body's immune system reacts abnormally with the allergen. The allergen binds to allergic antibodies (immunoglobulin E) that are attached to cells that produce histamine and other chemicals. The pollen "triggers" these cells in the nasal membranes, causing them to release histamine and the other chemicals. Histamine dilates the small blood vessels of the nose and fluids leak out into the surrounding tissues, causing runny noses, watery eyes, itching, swelling and other allergic symptoms.
Antibodies circulate in the blood stream, but localize in the tissues of the nose and in the skin. This makes it possible to show the presence of these antibodies by skin testing, or less commonly, by a special blood test. A positive skin test mirrors the type of reaction going on in the nose.