An allergy is a state of special sensitivity to a particular environmental substance, or allergen. An allergic reaction is the body's response to exposure to an allergen.
Although an allergy can be present almost immediately after exposure to an allergen, it usually develops over time, as the immune system forms antibodies against the foreign substance. Under normal conditions, such antibodies work to protect the body from further attack. In the case of an allergy, however, the antibodies and other specialized cells involved in this protective function trigger an unusual sensitivity, or overreaction, to the foreign substance.
The antibodies stimulate specialized cells to produce histamine, a powerful chemical. Histamine causes the small blood vessels to enlarge and the smooth muscles (such as those in the airways and the digestive tract) to constrict. Histamine release can also cause other reactions, such as hives.
No one knows why allergies develop, but it is known that an allergy can appear, disappear, or reappear at any time and at any age. Allergic reactions rarely occur during the first encounter with the troublesome allergen because the body needs time to accumulate the antibodies. Also, an individual's sensitivity to certain allergens seems to be related to a family history of allergies. People who have a tendency to develop allergies are referred to as atopic.
An allergic reaction can be so mild that it is barely noticeable or so severe that it is life-threatening. Common symptoms of allergy are watery eyes, runny nose, itching or inflamed skin, and a swollen mouth or throat. Some allergic reactions may be accompanied by headaches, sinus stuffiness, a reduced sense of taste or smell, or difficulty breathing.
An extremely severe allergic reaction, called anaphylactic shock, is marked by breathing difficulties (from swelling of the throat and larynx and narrowing of the bronchial tubes), itching skin, hives, and collapse of the blood vessels, as well as by vomiting, diarrhea, and cramps. This condition can be fatal if not treated immediately.
Types of Allergens
There are four categories of allergens: inhalants, contactants, ingestants, and injectants.
Inhalant allergens are those that are breathed in, including such substances as dust, pollen, feathers, and animal dander (small scales from an animal's skin). Hay fever is an inhalant allergy in which the mucous membranes react to various inhaled substances, usually the pollens associated with the changing seasons. Year-round "hay fever" may actually be a reaction to pet dander, feathers, mold, or dust.
Substances you come in contact with that irritate the skin -- such as poison ivy, cosmetics, detergents, fabrics, and dyes -- cause contact dermatitis.
Ingestant allergens are those that are swallowed. A variety of foods and medications can act as ingestant allergens. Food allergies occur more frequently in children than they do in adults. Common ingestant allergens are milk, eggs, shellfish, fish, peanuts, chocolate, strawberries, tomatoes, and citrus fruits.
Injectant allergens are substances that penetrate the skin, such as insect venom and drugs that are injected. For example, people who have a severe allergic reaction to an insect bite or sting are suffering from a reaction to an injectant allergen.
Now that we know what allergens are and the types of allergens, let's consider how they are diagnosed, treated and prevented. We cover them in the next section.