The Basics of the Immune System:
The place to start when talking about allergies is the immune system. The immune system is the catch-all term for the various bodily systems that act to fight against disease and toxins. These include the white blood cells, or leukocytes; the skin and mucous membranes that act as physical barriers to germs; and the bone marrow and spleen that produce disease fighting cells called antibodies. When the body is exposed to germs, the immune system produces antibodies to the specific germs (or antigens) it was exposed to in order to fight them off in the future. These antibodies bind to antigens to attack them.
How the Immune System Works in a Food Allergy:
For the most part, this system works well. The body recognizes cells that are part of itself and cells that are not, and recognizes cells that are threatening (e.g., viruses and bacteria) and cells that are harmless (e.g., food and pollen). However, in allergic individuals, the immune system identifes a food as a germ or antigen and produces antibodies -- specifically, Immunoglobulin E, or IgE -- to it. A food that has been primed in this manner is called an allergen. The resulting IgE/mast cell complex recognizes and binds to allergens, and this reaction stimulates the release of histamine and other chemicals.
When the body is exposed to an allergen, Immunoglobulin IgE attaches to what it perceives as a foreign invader and releases a number of chemicals as a form of attack. The most well-known chemical, which is responsible for a host of allergic symptoms, released in this reaction is called histamine. (Many drugs that treat allergies are called antihistamines because they treat the effects of this chemical). Histamine induces tissue swelling and can cause a host of local and systemic symptoms, including hives, rhinitis, and vomiting.
Allergies vs. Intolerances:
An allergy describes a very specific response of the immune system. It does not describe every reaction a person can have to a food. Unpleasant reactions that do not involve IgE are known as food intolerances. Food intolerances often occur in the digestive tract rather than the immune system. It is important to note that the distinction between an allergy and an intolerance is not necessarily one of severity; some intolerances are more severe than some allergies.