An allergy occurs when the body reacts to substances it views as harmful. The substances, such as tree pollen or dust mites, are called allergens. Following the body’s first exposure to an allergen, white blood cells produce antibodies that prepare the immune system for the next encounter with the same allergen. Antibodies attach themselves to mast cells--special cells found in the tissues of the respiratory and digestive systems.
Subsequent exposure to even a small amount of the allergen triggers the mast cells to release histamine. When histamine is released, it attaches to the receptors of nearby cells. These receptors interact with other substances in the body and cause nearby blood vessels to swell and secrete excess fluid-causing symptoms such as sneezing, runny nose, and watery eyes.
Antihistamines are drugs that can protect cells from some of the allergic effects caused by histamine. By attaching themselves to the receptors, these drugs can prevent cell receptors from binding to histamine. This prevents histamine from causing the chemical reactions in the cells that produce allergy symptoms.