"An allergy is the body's reaction to a normally harmless substance," says Dr. Stokes of Creighton University in Omaha, Nebraska. The first time an allergy-prone person comes into contact with these innocuous molecules, called allergens, her immune system will produce large quantities of allergen-specific antibodies called Immunoglobulin E (IgE). The antibodies attach themselves to white blood cells called mast cells and basophils; the next time the immune system detects that specific allergen, the IgE antibodies tell the white blood cells to release invader-fighting molecules, such as histamines. The molecules attach to different cell receptors in the body to trigger an inflammatory response that results in localized allergy symptoms. The severity of the symptoms depends on the type of allergy, the concentration of allergens you've taken in, and the route the allergens took to get into the body.
It's not completely understood what causes a person to develop allergies, but scientists know there's a very strong genetic component to it. "The greatest risk factor for developing allergies is family history," says Dr. Riedl at the University of California, San Diego. But there are many other risk factors for allergies, including C-sections and the environment you are raised in (research suggests that kids raised in cities are more prone to food allergies than their rural counterparts).