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 treats the substance, called an allergen , as an invader. With each subsequent exposure, the body produces more IgE, which attaches itself to cells in the body called mast cells. 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 airborne allergens such as pollen and dust mites, animals, and foods.
Allergies can be seasonal, like pollen or certain molds, or year-round, like dust mites.
Regional differences also occur. Different allergens are more prevalent in different parts of the country or the world. For example, peanut allergy is unknown in Scandinavia, where they do not eat peanuts, but common in the U.S., where the per capita consumption of peanuts is about eight pounds a year. Children inherit allergic tendencies from their parents. If one parent has allergies, there is a one in four chance that a child will also have allergies.