Allergy is an abnormal reaction of the body to substances normally harmless, such as pollen, dust mites, certain foods, drugs,
and insect stings. The term allergy is of Greek origin and means "abnormal response." An estimated 35 million people in the United States suffer from various allergies, some of which are mistaken for the common cold.
The
symptoms of allergy vary with the causative agent, which is called an allergen (or antigen), and with the part of the body affected. The symptoms, or allergic reactions, may include sneezing, watery eyes, and nasal congestion, as in hay fever and allergic rhinitis; a rash, stomach upset, and itchy swellings on the skin (hives), as in some food or drug allergies; spasms within the lungs that interfere with breathing, as in asthma. Rarely, a person may have an extreme allergic reaction to bee or wasp stings, penicillin or other drugs, or certain foods. This is called anaphylactic shock. Characterized by a severe drop in blood pressure, an itchy rash or hives, trouble breathing, pain in the abdomen, swelling of the tongue or throat, and diarrhea, it can lead to asphyxiation and death.
Common
allergens, in addition to those mentioned above, include animal dander, feathers, cosmetics, textile dyes, poison ivy and other plants, molds, animal excretions, and blood serum received by transfusion, which may cause serum sickness. Heat, cold, and light may cause hives in susceptible people. Allergens may act following inhalation, injection, ingestion, or contact with the skin.
THE ALLERGIC REACTION
An allergic reaction occurs when the immune system, which is the body's normal defense against dangerous foreign substances, "mistakes" a normally harmless substance for an invader, such as a virus. No one knows why this abnormal reaction occurs in some people and not others. People who have this type of unusual immune system are said to be hypersensitive, and medical scientists often use the term hypersensitivity instead of allergy.
The body's immune system reacts to an allergen in many different ways to cause the discomforting symptoms of an allergy. The process begins when the allergen stimulates the immune system to manufacture certain antibody molecules called immunoglobulin E (IgE). The antibody molecules then bind to receptors on mast cells and basophils. Upon combination of bound IgE antibodies with allergens, the cells release histamine (see antihistamine) and other active compounds. The histamine in turn affects the blood vessels and mucous membranes, leading to swelling, congestion, and leakage. Typically, these physiological changes lead to a runny nose or airway spasms if the allergen is airborne and inhaled. Other cells and other constituents of the blood serum can cause other types of allergies, such as poison ivy dermatitis and serum sickness.
TYPES OF ALLERGY
Characteristically people are not symptomatic with the first exposure to the allergen; symptoms occur only upon reexposure to the same agent. A person is said to have been sensitized by the first contact. That is, the immune system somehow "learns" to respond to the agent with an allergic reaction, but it reacts only at contacts that occur later. Sensitization of this type occurs in a variety of infectious diseases, such as farmer's lung, aspergillosis, and coccidioidomycosis, in which allergy develops to the infecting bacteria or fungi.
In many persons, heredity is responsible for the tendency to be allergic to a variety of substances. Persons with allergies are often called atopic. The common atopies include hay fever; asthma; infantile eczema, which is an itchy skin lesion; contact dermatitis, which is a skin inflammation caused by poison ivy or a variety of chemicals that may contact the skin; and perhaps some food or drug allergies. Numerous studies have shown that persons with one of these diseases are more likely to have other diseases of the group than is the general population. Many of these people have amily history of allergic diseases, usually of the same group.
DIAGNOSIS
Allergic disease is diagnosed from the patient's medical history; symptoms; skin, or patch, tests; and a type of blood test called radio allergo sorbent test (RAST), which help to identify the allergen. Small doses of many of the most common allergens are injected just below or placed on the skin. Substances to which the patient is allergic usually cause redness and swelling at the injection site. RAST determines the amount of IgE against specific antigens.
TREATMENT
If the allergens can be identified, treatment of allergy may be merely the avoidance of the offending agents. If they cannot be avoided, as with house dust mites, pollen, insect stings, and perhaps animal dander, treatment may be desensitization or immunotherapy. In desensitization, small amounts of the substance that causes the allergy are injected under the skin during repeated visits to a physician. After many such injections, the body may "learn" not to react to the substance. With the determination in 1988 of the structure of the IgE receptors on mast cells, researchers hope to develop more direct means of blocking allergic reactions.
The discomforting symptoms of allergy are sometimes relieved by four types of medication: antihistamines, which block the effects of histamines that cause congestion of mucous membranes and swelling; antiinflammatory agents, such as corticosteroids; decongestants, of short-term help; and cromolyn sodium, which blocks the allergic reaction.