Think of an allergy as a valuable immune response in the body gone haywire. Normally, the human body produces antibodies to germs or toxins. These antibodies attack invaders and kill viruses, bacteria, and parasites, performing a valuable function and keeping the body safe.
However, in the case of an allergy, substances that are not germs or poisons trigger the same response. The antibodies produced in response to allergens are usually IgE antibodies, which cause certain cells to release histamine, a protein that can cause a variety of classic allergy symptoms. In the case of food allergies, these antibodies generally cause one of a cluster of well-understood symptoms. It's important to note that people can develop allergies to any number of substances beyond foods --- allergies to molds, animal danders, pollens, and other substances follow the same biochemical pathways in the body.