Definition

Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.

Description

Allergies are among the most common of medical disorders. About one quarter of all Americans suffer from some form of allergy, such as asthma, allergic rhinitis (hay fever), or atopic dermatitis (eczema). Allergy is the single largest reason for school absence; allergic rhinitis alone is responsible for two million school day absences annually. Allergies cause one out of nine physician visits and are responsible for significant losses of productivity in the workplace.

An allergy is a type of immune reaction. Normally, the immune system responds to bacteria, viruses, or particles—such as pollen or dust—by producing antibodies (specific proteins) capable of binding to antigens (identifying molecules) on the foreign particle. The interaction between the antibody and antigen sets off a series of reactions designed to protect the body from infection. When this same series of reactions is triggered by harmless, everyday substances, it is known as an allergy, and the offending substance is called an allergen.

Allergens enter the body through four main routes: the airways, skin, gastrointestinal tract, and the circulatory system.

• Airborne allergens cause the sneezing, runny nose of allergic rhinitis. 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 may cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause urticaria (hives) or more severe reactions such as 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 a sudden drop in blood pressure.
• Allergins that come in direct contact with the skin can cause reddening, itching, and blistering (contact dermatitis). Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This reaction is known as atopic dermatitis or eczema.
• Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulatory system. There they may cause systemwide responses (including anaphylaxis), as well as local reactions of swelling and irritation at the injection site.

Individuals with allergies are not equally sensitive to all allergens. For example, some may have severe allergic rhinitis but no food allergies, others are extremely sensitive to nuts but not to any other food. Allergies may worsen over time. For example, childhood ragweed allergy may progress to year-round dust and pollen allergy. On the other hand, an individual may lose allergic sensitivity. Infant or childhood atopic dermatitis almost always disappears with advancing age. More commonly, an individual's apparently diminished sensitivity may instead be attributable to reduced exposure to allergens or an increased tolerance for allergy symptoms.

Causes and symptoms

Causes

Mast cells involved in allergic reactions capture and display an antibody, called immunoglobulin E (IgE), that binds to allergens. After the allergen is bound, mast cell granules release a variety of potent chemicals, including histamine, that are responsible for some of allergic symptoms.

Immunologists distinguish allergic reactions into two main types: immediate hypersensitivity reactions, which are mainly mast cell-mediated and occur within minutes of contact with allergen, and delayed hypersensitivity reactions, mediated by T cells (a type of white blood cells) and occurring hours to days after exposure.

Inhaled or ingested allergens usually cause immediate hypersensitivity reactions. Allergens bind to IgE antibodies on the surface of mast cells, which release the contents of their granules onto neighboring cells, including blood vessels and nerve cells. Histamine binds to the surfaces of these other cells through special proteins called histamine receptors. Interaction of histamine with receptors on blood vessels causes increased leakage, thereby producing fluid collection, swelling, and redness. Histamine also stimulates pain receptors, making tissue more sensitive and irritable. Symptoms last from one to several hours following contact.

In the upper airways and eyes immediate hypersensitivity reactions cause the runny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract these reactions lead to swelling and irritation of the intestinal lining, causing the cramping and diarrhea typical of food allergy. Allergens that enter the circulatory system may cause hives, angioedema, anaphylaxis, or atopic dermatitis.

Allergens on the skin usually cause delayed hypersensitivity reaction. Roving T cells contact the allergen, setting in motion a more prolonged immune response. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.

THE ROLE OF INHERITANCE. While allergy to specific allergens is not inherited, the likelihood of developing some type of allergy seems to be, at least for many people. If neither parent has allergies, then the chances of a child developing allergy is approximately 10–20%; when one parent has allergies, it is 30–50%; and when both have allergies, it is 40–75%. Allergy patients share a genetic predisposition to produce higher levels of IgE in response to allergens. Those who produce more IgE will develop a stronger allergic sensitivity.

COMMON ALLERGENS. The most common airborne allergens are the following:

• plant pollens
• animal fur and dander
• body parts from house mites (microscopic creatures found in all houses)
• house dust
• mold spores
• cigarette smoke
• solvents
• cleaners

Common food allergens include the following:

• nuts, especially peanuts, walnuts, and Brazil nuts
• fish, mollusks, and shellfish
• eggs
• wheat
• milk
• food additives and preservatives

The following types of drugs commonly cause allergic reactions:

• penicillin or other antibiotics
• flu vaccines
• tetanus toxoid vaccine
• gamma globulin

Common causes of contact dermatitis include:

• poison ivy, oak, and sumac
• nickel or nickel alloys
• latex

Insects and other arthropods whose bites or stings typically cause allergy include:

• bees, wasps, and hornets
• mosquitoes
• fleas
• scabies

Symptoms

Symptoms depend on the specific type of allergic reaction. Allergic rhinitis is characterized by an itchy, runny nose, often with a scratchy or irritated throat due to post-nasal drip. Allergic conjunctivitis (inflammation of

the thin membrane covering the eye) causes redness, irritation, and increased tearing in the eyes. Asthma causes wheezing, coughing, and shortness of breath. Symptoms of food allergies depend on the tissues most sensitive to the allergen and whether it is spread systemically by the circulatory system. Gastrointestinal symptoms may include swelling and tingling in the lips, tongue, palate or throat, and nausea, cramping, diarrhea, and gas. Contact dermatitis is marked by reddened, itchy, weepy skin blisters.

Systemic reactions may occur from any type of allergen, but are more common following ingestion or injection of an allergen. Skin reactions include hives and angioedema (a deeper and more extensive skin reaction) involving more extensive fluid collection. Anaphylaxis is marked by airway constriction, blood pressure drop, widespread tissue swelling, heart rhythm abnormalities, and, in some cases, loss of consciousness.

Diagnosis

Allergies may often be diagnosed by taking a detailed medical history, matching the onset of symptoms to the exposure to possible allergens. Allergy tests may be used to identify potential allergens. These tests usually begin with prick tests or patch tests that expose the skin to small amounts of allergen to observe the response. Reaction will occur on the skin even if the allergen is normally encountered in food or in the airways.

RAST testing, performed by a laboratory technologist, is a blood test that measures the level of reactive IgE antibodies in the blood. Provocation tests, most commonly done with airborne allergens, present the allergen directly through the route normally involved. Food allergen provocation tests require abstinence from the suspect allergen for two weeks or more, followed by ingestion of a measured amount.

Treatment

A variety of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most work by decreasing the ability of histamine to provoke symptoms. Other drugs counteract the effects of histamine by stimulating other systems or reducing immune responses in general.

Drugs

ANTIHISTAMINES. Antihistamines block the histamine receptors on nasal tissue, decreasing the effect of histamine released by mast cells. They may be used after symptoms appear, though they may be even more effective when used before symptoms appear. A wide variety of antihistamines are available.

Some antihistamines produce drowsiness as a major side effect. These include:

• diphenhydramine (Benadryl and generics)
• chlorpheniramine (Chlor-trimeton and generics)
• brompheniramine (Dimetane and generics)
• clemastine (Tavist and generics)

Antihistamines that do not cause drowsiness are available by prescription and include the following:

• astemizole (Hismanal)
• loratidine (Claritin)
• fexofenadine (Allegra)
• azelastin HCl (Astelin)

Hismanal has the potential to cause serious heart arrhythmia when taken with the antibiotic erythromycin, the antifungal drugs ketoconazole and itraconazole, or the antimalarial drug quinine. Exceeding the recommended dose of Hismanal may also cause arrhythmia.

DECONGESTANTS. Decongestants constrict blood vessels to counteract the effects of histamine. Nasal sprays, 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. Topical corticosteroids reduce mucous membrane inflammation and are available by prescription. Allergies tend to worsen as the season progresses because the immune system becomes sensitized to particular antigens and 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.

MAST CELL STABILIZERS. 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 also may 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

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. Injections involve very small but gradually increasing amounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take as long as several years to achieve, and are not seen at all in about one in five patients. Patients are monitored closely following each shot because of the small risk of anaphylaxis.

Bronchodilators

Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow (as in asthma), bronchodilators, which cause the smooth muscle lining the airways to dilate, can be very effective. Some bronchodilators used to treat acute asthma attacks include adrenaline, albuterol, or other adrenergic stimulants, most often administered as aerosols. Theophylline, naturally present in coffee and tea, is another drug that produces bronchodilation. It is usually taken orally, but in a severe asthma attack is may be administered intravenously. Other drugs, including steroids, are used to prevent asthma attacks and in the long-term management of asthma.

Treatment of contact dermatitis

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

Treatment of anaphylaxis

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

Prognosis

Allergies may 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 patients with allergies to lead normal lives.

Health care team roles

Diagnosis and effective management of allergy symptoms involves cooperation and collaboration between the patient and an interdisciplinary team of health care professionals. The patient's primary care physician or pediatrician, allergy and immunology specialists, laboratory technologists, respiratory therapists, pharmacists, pharmacy assistants, and health educators are involved in helping patients and families gain an understanding of how to prevent effectively manage symptoms.

Patient education

Nurses, respiratory therapists, and health educators help patients learn how to prevent and manage allergy symptoms. They teach patients how to distinguish mild allergy symptoms from those requiring immediate medical attention. Pharmacists and pharmacy assistants may offer additional instruction about medication use and reiterate the importance of adhering to prescribed treatment.

Prevention

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 cause reactions, most patients can learn to avoid allergic reactions from food, drugs, and contact allergens such as poison ivy or latex. Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure. Cromolyn sodium can prevent mast cell degranulation, thereby limiting the allergic response.

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KEY TERMS

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Allergen—A substance that provokes an allergic response.

Allergic rhinitis—Inflammation of the mucous membranes of the nose and eyes in response to an allergen; also known as hay fever.

Anaphylaxis—Increased sensitivity caused by previous exposure to an allergen that can result in blood vessel dilation and smooth muscle contraction. Anaphylaxis can result in sharp blood pressure drops and difficulty breathing.

Angioedema—Severe non-inflammatory swelling of the skin, organs, and brain that can also be accompanied by fever and muscle pain.

Antibody—A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.

Antigen—A foreign protein to which the body reacts by making antibodies.

Asthma—A lung condition where the airways become constricted due to smooth muscle contraction, causing wheezing, coughing, and shortness of breath.

Atopic dermatitis—Inflammation of the skin as a result of exposure to airborne or food allergens; also known as eczema.

Conjunctivitis—Inflammation of the thin lining of the eye called the conjunctiva.

Contact dermatitis—Inflammation of the skin as a result of contact with a substance.

Delayed hypersensitivity reactions—Allergic reactions mediated by T cells that occur hours to days after exposure.

Granules—Small packets of reactive chemicals stored within cells.

Histamine—A chemical released by mast cells that activates pain receptors and causes cells to become leaky.

Immune hypersensitivity reaction—Allergic reactions that are mediated by mast cells and occur within minutes of allergen contact.

Mast cells—A type of immune system cell found in the lining of the nasal passages and eyelids, with an antibody called immunoglobulin type E (IgE) on its cell surface; mast cells release histamine from intracellular granules.

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Barbara Wexler