Allergies are abnormal reactions of the immune system which occur in responseto otherwise harmless substances.

Allergies are among the most common of medical disorders. It is estimated that 60 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. Allergy is the single largest reason for school absence and is a major source of lost productivity in the workplace.

An allergy is a type of immune reaction. Normally, the immune system respondsto foreign microorganisms or particles, like pollen or dust, by producing specific proteins called antibodies that are capable of binding to identifyingmolecules, or antigens, on the foreign particle. This reaction between antibody and antigen sets off a series of reactions designed to protect the body from infection. Sometimes, this same series of reactions is triggered by harmless, everyday substances. This is the condition known as allergy, and the offending substance is called an allergen.

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

  • Airborne allergens cause the sneezing, runny nose, and itchy, bloodshot eyes of hay fever (allergic rhinitis). Airborne allergens can also affect the lining of the lungs,causing asthma, or the conjunctiva of the eyes, causing conjunctivitis (pinkeye).
  • Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they maycause hives (urticaria) or more severe reactions involving recurrent, non-inflammatory 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.
  • In contact with the skin, allergens can cause reddening,itching, and blistering, called contact dermatitis. Skin reactions can alsooccur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis.
  • Injection ofallergens, from insect bites and stings or drug administration, can introduceallergens 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.

People with allergies are not equally sensitive to all allergens. Some may have severe allergic rhinitis but no food allergies, for instance, or be extremely sensitive to nuts but not to any other food. Allergies may get worse overtime. For example, childhood ragweed allergy may progress to year-round dustand pollen allergy. On the other hand, a person may lose allergic sensitivity. Infant or childhood atopic dermatitis disappears in almost all people, forexample. More commonly, what seems to be loss of sensitivity is instead a reduced exposure to allergens or an increased tolerance for the same level of symptoms.

Mast cells, one of the major players in allergic reactions, capture and display a particular type of antibody, called immunoglobulin type E (IgE) that binds to allergens. Inside mast cells are small chemical-filled packets called granules. Granules contain a variety of potent chemicals, including histamine.

Immunologists separate allergic reactions into two main types: immediate hypersensitivity reactions, which are mainly mast cell-mediated and occur withinminutes 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 spill the contents of their granules out onto neighboring cells, including bloodvessels 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 leakiness, leading to thefluid collection, swelling and increased redness. Histamine also stimulatespain 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 therunny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract, these reactions lead to swelling and irritation of theintestinal lining, which causes the cramping and diarrhea typical of food allergy. Allergens that enter the circulation may cause hives, angioedema, anaphylaxis, or atopic dermatitis.

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

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 neitherparent has allergies, the chances of a child developing allergy is approximately 10-20%; if one parent has allergies, it is 30-50%; and if both have allergies, it is 40-75%.One source of this genetic predisposition is in the ability to produce higher levels of IgE in response to allergens. Those who producemore IgE will develop a stronger allergic sensitivity.

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
  • Foodadditives 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 the following:

  • Poison ivy,oak, and sumac
  • Nickel or nickel alloys
  • Latex.

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

  • Bees, wasps, and hornets
  • Mosquitoes
  • Fleas
  • Scabies.

Symptoms depend on the specific type of allergic reaction. Allergic rhinitisis characterized by an itchy, runny nose, often with a scratchy or irritatedthroat due to post-nasal drip. Inflammation of the thin membrane covering theeye (allergic conjunctivitis) 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 allergenand whether it is spread systemically by the circulatory system. Gastrointestinal symptoms may include swelling and tingling in the lips, tongue, palateor throat; nausea; cramping; diarrhea; and gas. Contact dermatitis is markedby reddened, itchy, weepy skin blisters.

Whole body or systemic reactions may occur from any type of allergen, but aremore common following ingestion or injection of an allergen. Skin reactionsinclude the raised, reddened, and itchy patches called hives. A deeper and more extensive skin reaction, involving more extensive fluid collection, is called angioedema. Anaphylaxis is marked by airway constriction, blood pressuredrop, widespread tissue swelling, heart rhythm abnormalities, and in some cases, loss of consciousness.

Allergies can often be diagnosed by a careful medical history, matching the onset of symptoms to the exposure to possible allergens. Allergy tests can beused to identify potential allergens. These tests usually begin with prick tests or patch tests, which 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 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. Provocation tests are not used if anaphylaxis isis a concern due to the patient's medical history. For a complete description, see the article on allergy testing.

A large number of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most of these 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.

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

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)
  • Clemastine (Tavist and generics).

Newer 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 arrhythmias when taken withthe antibiotic erythromycin, the antifungal drugs ketoconazole and itraconazole, or the antimalarial drug quinine. Taking more than the recommended doseof 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.

Decongestants constrict blood vessels to counteract the effects of histamine.Nasal sprays are available that can be applied directly to the nasal liningand 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 ofeffectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment.

Topical corticosteroids reduce mucous membrane inflammation and are availableby prescription. Allergies tend to become worse 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.

Cromolyn sodium prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. It acts asa 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 inthe 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 willbe monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop inblood pressure.

Because allergic reactions involving the lungs cause the airways or bronchialtubes to narrow, as in asthma, bronchodilators, which cause the smooth 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 brochodilation. It is usually taken orally, but in a severe asthma attack is maybe given intravenously. Other drugs, including steroids, are used to preventasthma 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 dryand scaly skin.

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

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 foodallergies, there is no effective treatment except avoidance. By determiningthe allergens that are causing reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens such as poison ivy orlatex. Airborne allergens are more difficult to avoid, although keeping dustand 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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