The Lymph System

Production of white blood cells and antibodies in reaction to an invading disease organism is called an immune response. This response is one of the body's primary and most efficient lines of defense. In most cases, once antibodies have been produced to fight a certain organism, it no longer poses a great threat to the body. That is why one attack of a disease often prevents that same disease from infecting the body again -- the first attack causes production of antibodies that protect the body against subsequent attacks. With measles, for example, antibodies are produced as a result of having the disease or of being immunized with the measles vaccine. These antibodies are able to resist a second attack of the disease.

Antibodies are not always beneficial. For example, when tissue from another body, such as a transplanted heart, is introduced, antibodies are produced to destroy the "invader." Transplants usually are made possible only by means of drugs that act against the body's natural immune response. Also, when blood is transfused from one person to another, it must be of a matching type; otherwise, the recipient's immune system will manufacture antibodies to destroy the transfused blood.

Sometimes, the immune system causes reactions that make the body unusually sensitive to foreign material. When the immune response is disruptive to the body in this way, it is called an allergic reaction. Let's look at this important mechanism, and the types of allergens, in the next section.

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.

Identifying an offending allergen may be uncomfortable, time-consuming, and expensive, but it is sometimes necessary in order to avoid future allergic reactions. A medical history, a record of any recent changes in daily habits, skin-scratch tests (in which small amounts of suspected allergens are applied to tiny scratches in the skin), and intracutaneous skin tests (in which allergens are injected under the skin) are used to help identify the troublesome foreign substance. A special study called radioallergosorbent testing (RAST) is sometimes performed to detect and measure antibodies in the blood that have been manufactured in response to invading substances.

Treatment

Once the allergen has been identified, half the battle is won. Avoiding the troublesome substance is the first step toward relieving an allergy problem. If the allergen cannot be avoided or removed, medication or immunotherapy may be recommended. Three types of medications are commonly prescribed: antihistamine drugs, which combat the effect of the histamines in the body; corticosteroids, which reduce inflammation and swelling; and bronchodilators, which ease breathing by opening bronchial tubes.

Allergy immunotherapy, or desensitization, consists of injections of an allergen in gradually increasing quantities. This allows the body to build up a tolerance to the offending substance. Immunotherapy works best in controlling allergies to pollen, insect venom, and house dust.

A little common sense goes a long way toward controlling allergies. Obviously, the offending allergen needs to be avoided. Keep windows shut during pollen season. Bathe your pet frequently if you are allergic to animal dander. Replace natural fibers in the home with synthetics. Air conditioners or air filters can be installed for hay fever sufferers. Those susceptible to insect bites can wear protective clothes and avoid wearing bright clothing and perfumes, both of which attract insects. Babies born into allergy-prone families should be breast-fed as long as possible to delay exposure to cow's milk, eggs, and citrus fruits.

Prevention

In most cases, allergies cannot be prevented, but much can be done to help control or diminish their effects on overly sensitive individuals.

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