What are allergies?
Allergies are hypersensitivities, overreactions of the immune system to substances that do not cause reactions in most people. Hypersensitivities are grouped into four types, I through IV. These classifications are based, to some extent, on what parts of the immune system are activated and how long it takes for a reaction to occur.
The two types of hypersensitivities commonly associated with the term "allergies" are type I immediate hypersensitivities, in which antigens (allergens, foreign substances) combine with specific IgE (immunoglobulin E) antibodies to cause local and sometimes systemic reactions, usually within minutes; and type IV delayed hypersensitivities, reactions caused by the interactions of antigens with specific sensitized T lymphocytes instead of antibodies.
According to the American Academy of Allergy Asthma and Immunology (AAAAI), as many as 50 million people in the United States have some type of allergy and the number appears to be increasing. While anyone can develop an allergy, those with affected family members are at an increased risk. A person who is "predisposed" may not, however, react to the same substances as his parents and siblings. It depends upon what antigens he is exposed to and his immune system's response.
Type I hypersensitivities primarily affect the respiratory and gastrointestinal systems and the skin. The first time a predisposed person is exposed to a potential allergen, they will not have a major reaction; instead, they will begin producing a specific IgE antibody and become "sensitized." Once someone is sensitized, subsequent exposures can result in severe reactions.
The IgE antibody produced attaches itself to mast cells, specialized cells in the tissues, and basophils in the bloodstream. This action primes the immune system. During subsequent exposures to the allergen, the specific bound IgE identifies the intruder, attaches to it, and triggers the release of chemicals, including histamine, causing allergic Symptoms that start in the mouth, nose, or on the skin, wherever the allergen has been introduced.
Type IV hypersensitivities usually involve the skin and are defined as "delayed" hypersensitivities since the reaction typically appears about 48-72 hours after exposure. These reactions occur when an antigen interacts with specific sensitized T lymphocytes. The lymphocytes release inflammatory and toxic substances, which attract other white blood cells to the exposure site, resulting in tissue injury. No immune system "priming" is necessary; people can have a type IV reaction with the first exposure. A common example of this type of allergy is the reaction to poison ivy.
What is not an allergy?
There are other reactions that can cause allergy-like symptoms but are not caused by an activation of the immune system. They range from toxic reactions that affect anyone who has sufficient exposure, such as food poisoning caused by bacterial toxins, to genetic conditions, such as intolerances caused by the lack of an enzyme - for example, the inability to digest milk sugar, resulting in lactose intolerance, and sensitivities to things like gluten (in Celiac disease). Symptoms can also be caused by medications such as aspirin and ampicillin, food dyes, MSG (monosodium glutamate – a food flavor additive), and by some psychological triggers. While these diseases and conditions may need to be investigated by a physician, they are not allergies and will not be identified during allergy testing.