Allergy

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Allergy

An allergy is an excessive or hypersensitive response of the immune system. The allergic reaction becomes manifest as a pathological immune reaction induced either by antibodies (immediate hypersensitivity) or by lymphoid cells (delayed type allergy). Instead of fighting off a disease-causing foreign substance, the immune system launches a complex series of actions against an irritating substance, referred to as an allergen. Allergens enter the body through four main routes: the airways, the skin, the gastrointestinal tract, and the circulatory system. Some problems include:

Airborne allergens cause the sneezing, runny nose, and itchy, bloodshot eyes of hay fever (allergic rhinitis). Some of the most common airborne allergens are plant pollens, animal fur and dander, body parts from house mites (microscopic creatures found in all houses), house dust, mold spores, cigarette smoke, solvents, and cleaners. Airborne allergens can also affect the lining of the lungs, causing asthma, or the conjunctiva of the eyes, causing conjunctivitis (pink eye).
Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause hives (urticaria) or more severe reactions involving recurrent, noninflammatory swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and drop in blood pressure. Common food allergens include nuts, especially peanuts, walnuts, and Brazil nuts; fish, mollusks, and shellfish; eggs; wheat; milk; food additives and preservatives.
In contact with the skin, allergens can cause reddening, itching, and blistering, called contact dermatitis. Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis.
Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulation, where they may cause system-wide responses (including anaphylaxis), as well as the local ones of swelling and irritation at the injection site.

The symptoms of an immediate hypersensitivity begin shortly after contact and decay rapidly, while the delayed type symptoms do not reach their maximum for 24 to 48 hours and decline slowly over a period of days or weeks. An allergic reaction may be accompanied by a number of stressful symptoms, ranging from mild to severe to life threatening.

In rare cases, an allergic reaction can lead to anaphylactic shock—a condition characterized by a sudden drop in bloodpressure, difficulty in breathing, skin irritation, collapse, and possible death.

Allergies are among the most common of medical disorders. According to the American Academy of Allergy, Asthma and Immunology, as of 2005, over 50 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergies, as a group, are the single largest reason for school absence and is a major source of lost productivity in the workplace.

The immune system may produce several chemical agents that cause allergic reactions. The main group of immune system substances responsible for the symptoms of allergy includes the histamines, which are produced after an exposure to an allergen. Along with other treatments and medicines, the use of antihistamines helps to relieve some of the symptoms of allergy by blocking out histamine receptor sites. The study of allergy medicine includes the identification of the different types of allergy, immunology, and the diagnosis and treatment of allergy.

Types of allergy

The most common cause of allergy is pollens that are responsible for seasonal or allergic rhinitis. The popular name for rhinitis, hay fever, a term used since the 1830s, is inaccurate because the condition is not characterized by fever. Throughout the world during every season, pollens from grasses, trees, and weeds affect certain individuals, producing allergic reactions like sneezing, runny nose, swollen nasal tissues, headaches, blocked sinuses, and watery, irritated eyes. According to the American Academy of Allergy, Asthma, and Immunology (AAAAI), of the 40 to 50 million allergy sufferers in the United States (as of 2006), about two-thirds of them have rhinitis. Allergic diseases affect over 20% of the population in the United States, making it the sixth leading cause of chronic disease in the country.

Dust and the house dust mite constitute another major cause of allergies. While the mite itself is too large to be inhaled, its feces are about the size of pollen grains and can lead to allergic rhinitis. Other types of allergy can be traced to the fur of animals and pets, food, drugs, insect bites, and skin contact with chemical substances or odors. In the United States, there are over 12 million people who are allergic to a variety of chemicals. In some cases an allergic reaction to an insect sting or a drug reaction can cause sudden death. Serious asthma attacks are associated with seasonal rhinitis and other allergies. According to the AAAAI, about 20 million people in the United States suffer from asthma in any given year.

Role of immune system

Some people are allergic to a wide range of allergens, while others are allergic to a few or only one— while some people have no allergies. The reasons for these differences can be found in the makeup of an individual’s immune system. The immune system is the body’s defense against substances (antigens) that it recognizes as foreign to the body. Lymphocytes, a type of white blood cell, produce antibodies, which play an important role in neutralizing antigens, including bacteria, viruses, and toxins. Those antigens specifically producing an allergic response are known as allergens When an allergen first enters the body, the lymphocytes produce an antibody called immunoglobulin E (IgE). The IgE antibodies attach to mast cells, large cells that are found in connective tissue and contain histamines along with a number of other chemical substances.

Studies show that allergy sufferers produce an excessive amount of IgE, indicating a hereditary factor for their allergic responses. How individuals adjust over time to allergens in their environments also determines their degree of susceptibility to allergic disorders.

The second time any given allergen enters the body, it becomes attached to the newly-formed Y-shaped IgE antibodies. These antibodies, in turn, stimulate the mast cells to discharge its histamines and other anti-allergen substances. There are two types of histamine: H₁ and H₂. H₁ histamines travel to receptor sites located in the nasal passages, respiratory system, and skin, dilating smaller blood vessels and constricting airways. The H₂ histamines, which constrict the larger blood vessels, travel to the receptor sites found in the salivary and tear glands and in the stomach’s mucosal lining. H₂ histamines play a role in stimulating the release of stomach acid, thus creating a seasonal stomach ulcer condition.

Diagnosis and treatment

The patient’s medical history provides the primary basis for the diagnosis of allergies. Skin patch tests are sometimes used to determine exactly which potential allergens can produce a reaction in the patient. A group of substances are placed in patches under the skin; any actual allergen will raise a weal (a red, circular swelling) at the site of the patch. The weal is a circular area of swelling that itches and is reddened. The tests at times produce false positives, so that they cannot be relied on exclusively.

The simplest form of treatment is the avoidance of the allergic substance, but that is not always possible. In such cases, desensitization to the allergen is sometimes attempted by exposing the patient to slight amounts of the allergen at regular intervals.

Antihistamines, which are now prescribed and sold over the counter as a rhinitis remedy, were discovered in the 1940s. There are a number of different ones, and they either inhibit the production of hista-mine or block them at receptor sites. After the administration of antihistamines, IgE receptor sites on the mast cells are blocked, thereby preventing the release of the histamines that cause the allergic reactions. The allergens are still there, but the body’s protective actions are suspended for the period of time that the antihistamines are active. Antihistamines also constrict the smaller blood vessels and capillaries, thereby removing excess fluids. Recent research has identified specific receptor sites on the mast cells for the IgE. This knowledge makes it possible to develop medicines that will be more effective in reducing the symptoms of various allergies.

Older antihistamines often produce drowsiness as a major side effect. Such antihistamines include the following:

Diphenhydramine (Benadryl^® and generics),
Chlorpheniramine (Chlor-trimeton^® and generics),
Brompheniramine (Dimetane^® and generics), and
Clemastine (Tavist^® and generics).

Newer antihistamines that do not cause drowsiness and include the following:

Astemizole (Hismanal^®),
Loratidine (Claritin^®),
Fexofenadine (Allegra^®), and
Azelastin HCl (Astelin^®).

Hismanal^® has the potential to cause serious heart arrhythmias when taken with the antibiotic erythromycin, the antifungal drugs ketoconazole and itraconazole, or the antimalarial drug quinine. Taking more than the recommended dose of Hismanal^® can also cause arrhythimas. Seldane^® (terfenadine), the original non-drowsy antihistamine, was voluntarily withdrawn from the market by its manufacturers in early 1998 because of this potential and because of the availability of an equally effective, safer alternative drug, fexofenadine.

Corticosteroids are sometimes prescribed to allergy sufferers as anti-inflammatories. Decongestants can also bring relief, but these can be used for a short time only, since their continued use can set up a rebound effect and intensify the allergic reaction.

Decongestants constrict blood vessels to counteract the effects of histamine. Nasal sprays are available that can be applied directly to the nasal lining and oral systemic preparations are available. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, and agitation. Use of topical decongestants for longer than several days can cause loss of effectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment.

Topical corticosteroids reduce mucous membrane inflammation and are available by prescription. Allergies tend to become worse as the season progresses because the immune system becomes sensitized to particular antigens and, thus, it can produce a faster, stronger response. Topical corticosteroids are especially effective at reducing this seasonal sensitization because they work more slowly and last longer than most other medication types. As a result, they are best started before allergy season begins. Side effects are usually mild, but may include headaches, nosebleeds, and unpleasant taste sensations.

Cromolyn sodium prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. It acts as a preventive treatment if it is begun several weeks before the onset of the allergy season. It can also be used for year round allergy prevention. Cromolyn sodium is available as a nasal spray for allergic rhinitis and in aerosol (a suspension of particles in gas) form for asthma.

Immunotherapy, also known as desensitization or allergy shots, alters the balance of antibody types in the body, thereby reducing the ability of IgE to cause allergic reactions. Immunotherapy is preceded by allergy testing to determine the precise allergens responsible. (These tests are described in full in the article on allergy testing.) Injections involve very small but gradually increasing amounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take up to several years to achieve and are not seen at all in about one in five patients. Individuals receiving all shots will be monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.

Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow, as in asthma, bronchodilators, which cause the smooth

KEY TERMS

Allergen— An otherwise harmless substance that can cause a hypersensitive allergic response.

Anaphylactic shock— A violent, sometimes fatal, response to an allergen after initial contact.

Corticosteroids— Drugs that stimulate the adrenal gland and are highly effective in treating asthma and allergies but also have many side effects.

Decongestants— Drugs used for a short term to reduce mucous membrane congestion.

Histamines— A chemical released from cells in the immune system as part of an allergic reaction.

IgE— The chief immunoglobulin responsible for producing the compounds that cause allergic reactions.

Lymphocyte— A white blood cell that stimulates antibody formation.

Mast cell— A tissue white blood cell located at the site of small blood vessels.

Receptor sites— Places on the mast cell where Y-shaped antibodies stimulate histamine production.

Rhinitis— The common condition of upper respiratory tract inflammation occurring in both colds and allergy.

Weal— The reddened, itchy swelling caused by a skin patch test.

muscle lining the airways to open or dilate, can be very effective. Some bronchodilators used to treat acute asthma attacks include adrenaline, albuterol, or other adrenoceptor stimulants, most often administered as aerosols. Theophylline, naturally present in coffee and tea, is another drug that produces brocho-dilation. It is usually taken orally, but in a severe asthma attack is may be given intravenously. Other drugs, including steroids, are used to prevent asthma attacks and in the long-term management of asthma.

Calamine lotion applied to affected skin can reduce irritation somewhat. Topical corticosteroid creams are more effective, though overuse may lead to dry and scaly skin.

The emergency condition of anaphylaxis is treated with injection of adrenaline, also known as epinephrine. People who are prone to anaphylaxis because of food or insect allergies often carry an Epi-pen containing adrenaline in a hypodermic needle. Prompt injection can prevent a more serious reaction from developing.

Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most people with allergies to lead normal lives.

Avoiding allergens is the best means of limiting allergic reactions. For food allergies, there is no effective treatment except avoidance. By determining the allergens that are causing reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens such as poison ivy. Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure.

See also Antibody and antigen.

Resources

BOOKS

Adelman, Daniel C., and Thomas B. Casale. Manual of Allergy and Immunology: Diagnosis and Therapy. 4th ed. Philadelphia: Lippincott Williams & Wilkins Publishers, 2002.

Holgate, Stephen T., Martin K. Church, and Lawrence M. Lichtenstein. Allergy. Philadelphia, PA: Mosby Elsevier, 2006.

Jackson, Mark. Allergy: The History of a Modern Malady. London, UK: Reaktion, 2006.

Wolf, Raoul L. Essential Pediatric Allergy, Asthma and Immunology. New York: McGraw-Hill Medical Pub. Division, 2004.

OTHER

American College of Allergy, Asthma & Immunology. “ACAAI Online.” <http://www.acaai.org/> (accessed November 27, 2006)

National Institutes of Health. “The National Institute of Allergy and Infectious Diseases” (November 6, 2006) >http://www.niaid.nih.gov/default.htm> (accessed November 27, 2006).

Jordan P. Richman

The Gale Encyclopedia of Science