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. The symptoms of an immediate hypersensitivity begin shortly after contact and decay rapidly, while the delayed type symptoms do not reach their maximum for 24–48 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 blood pressure , difficulty in breathing, skin irritation, collapse, and possible death.

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. Of the 46 million allergy sufferers in the United States, about 25 million have rhinitis.

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 about 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. About nine million people in the United States suffer from asthma.

Role of immune system

Some people are allergic to a wide range of allergens, while others are allergic to only a few or none. 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 histamine 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.

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.

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.

Joneja, Janice Vickerstaff, and Leonard Bielory. UnderstandingAllergy, Sensitivity, & Immunity. New Brunswick, NJ: Rutgers University Press, 1990.

Steinman, Marion. A Parent's Guide to Allergies and Asthma. New York: Dell Publishing, 1992.

Organizations

American College of Allergy, Asthma & Immunology, 85 West Algonquin Rd., Suite 550, Arlington Heights, IL 60005. 1–800–842–7777. <http://allergy.mcg.edu> (October 19, 2002).

Other

National Institutes of Health. "The National Institue of Allergy and Infectious Diseases" <http://www.niaid.nih.gov/default.htm> (October 19, 2002).

Jordan P. Richman

KEY TERMS

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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.