The term “allergy” is frequently used, but is often poorly understood. Here, we describe the different types of reactions that constitute allergy and we attempt to put to rest some popular misconceptions.
What Is an Allergy?
Allergy is a term used to describe a reproducible immunologic response to a trigger. “Classic” allergy involves an Immunoglobulin E (IgE) antibody-dependent response to an allergen trigger. This response results in the release of chemical mediators, such as histamine, into the parts of the body that are exposed to the allergen, such as the skin, lungs, nose, circulation, leading to allergic symptoms, such as hives, wheezing, sneezing, or anaphylactic shock.
For example, in late summer, IgE antibody in the nose binds to ragweed pollen, leading to mediator release, which, in turn, leads to sneezing, itching, runny nose, and congestion. An important concept in allergy is that the individual with allergies has to have been previously exposed, in some manner, to a substance to acquire sensitivity to it. This is the reason why small babies are less likely to have some types of allergies than older children.
What Causes Allergy?
The answer to this question varies according to the particular allergic response. For example, respiratory tract responses, such as runny nose and wheezing, are usually caused by inhaled allergens, such as pollen, mold, pet dander, and dust mites.
Beesting allergy is caused by the injection, by a sting, of the venom of certain insects (see the beesting page for a thorough discussion). Drug allergy is caused by the internal exposure to a medication after it has been injested, injected, or otherwise given to a patient. Food allergy is caused by the injestion of a food to which an individual has become sensitive.
Other allergic responses, such as hives and other rashes, are sometimes caused by the classical IgE-mediated pathway and sometimes by other pathways (as discussed in Classical Allergy).
What About Allergy to Smoke and Perfumes?
There is an important distinction to be made between true allergy, that is, a reproducible response to an allergen, and sensitivity to irritants. In general, true allergens are biologic substances, such as pollens, foods, or medications whereas irritants are chemicals or toxins, such as perfumes, cigarette smoke, and auto exhaust. The distinction can sometimes be difficult to make, because the symptoms of a response to an allergen and a response to an irritant can be very similar.
One useful fact to keep in mind is that allergic responses are not dose-dependent while irritant responses are dose-dependent. For example, a very brief exposure to a cat can make someone sneeze for hours, but it may take a lengthy exposure to perfume or cigarette smoke to make an irritant-sensitive individual have sustained nasal symptoms.
For more information on the classical allergy response, see Classical Allergy.