According to the American Academy of Allergy, Asthma and Immunology (AAAAI), the percentage of people worldwide who suffer from some sort of allergy has risen steadily for the last 50 years with almost 50% of all children worldwide exhibiting some sort of allergic reaction to at least one type of food, insect, drug, vegetation, or animal.
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 or her parents and siblings. It depends upon what allergens the person is exposed to and that person's immune system's response.
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—people with these types of allergies produce immune proteins called antibodies when exposed to an allergen. The class of antibody produced is called immunoglobulin E (IgE). When these antibodies bind to the allergens, they cause local reactions and sometimes reactions affecting the whole body, usually within minutes.
- Type IV delayed hypersensitivities—these reactions are caused by the interactions of allergens with specific sensitized immune cells, white blood cells called T lymphocytes.
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, that person will not have a major reaction; instead, the person will begin producing IgE antibodies directed against the specific allergen (allergen-specific IgE antibody). Once someone is sensitized in this way, subsequent exposures can result in severe reactions.
The IgE antibody produced in response to an allergen binds 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 recognizes the allergen, binds to it, and triggers the release of chemicals, including histamine, causing allergic symptoms that can start in the mouth, nose, or on the skin, wherever the allergen has been introduced.
Type I allergic reactions can vary in severity, one time causing a red rash or hives, the next time a serious and life-threatening acute reaction called anaphylaxis. Anaphylaxis is a multi-organ reaction that can start with agitation, a feeling of "impending doom," pale skin due to low blood pressure, and/or a loss of consciousness (fainting). Anaphylaxis can be fatal without the rapid administration of an epinephrine (adrenaline) injection.
Type I allergies can be in response to a variety of substances, including but not limited to:
- Plants such as pollens, weeds, grasses, etc.
- Insect venoms
- Animal dander and saliva from cats and dogs, for example
- Dust mites
- Mold spores
- Occupational substances (latex)
- Drugs such as penicillin
There can also be cross-reactions, where someone allergic to ragweed, for instance, may also react to melons (watermelon or cantaloupe) and bananas. The most common food-related causes of severe anaphylactic reactions are peanuts, tree nuts such as walnuts, and shellfish.
Type IV hypersensitivities usually involve the skin and are defined as "delayed" hypersensitivities since the reaction typically appears about 48-72 hours after exposure to an allergen. These reactions occur when an allergen 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 health practitioner, they are not allergies and will not be identified during allergy testing.