An allergy is an overreaction of the immune system toward a substance that is typically harmless to most people. But in someone with an allergy, the body’s immune system treats the substance, called an allergen, as an invader and reacts inappropriately resulting in harm to the person.
In most common type of allergy, at the first exposure to an allergen, the immune system produces an antibody called immunoglobulin E (IgE). With each subsequent exposure, the body produces more IgE, which attaches itself to two types of cells in the body called mast cells and basophils. The allergen attaches to the IgE, and the mast cells and basophils are activated to release histamine and other chemicals to defend against the allergen “invader.” It is the release of these chemicals that causes allergic reactions, as the body attempts to rid itself of the invading allergen.
Some of the most common allergies include those to food and to airborne allergens such as pollen, mold, dust mites, and animal emanations. Allergies can be seasonal, like pollen or certain molds, or year-round, like dust mites. Regional differences also occur since different allergens are more prevalent in different parts of the country or the world.