Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance.
Normally, the immune system—which includes antibodies, white blood cells, mast cells, complement proteins, and other substances—defends the body against foreign substances (called antigens). However, in susceptible people, the immune system can overreact when exposed to certain substances (allergens) in the environment, foods, or drugs, which are harmless in most people. The result is an allergic reaction. Some people are allergic to only one substance. Others are allergic to many. About one third of the people in the United States have an allergy.
Allergens may cause an allergic reaction when they land on the skin or in the eye or are inhaled, eaten, or injected. An allergic reaction can occur in several ways:
In many allergic reactions, the immune system, when first exposed to an allergen, produces a type of antibody called immunoglobulin E (IgE). IgE binds to a type of white blood cell called basophils in the bloodstream and to a similar type of cell called mast cells in the tissues. The first exposure may make people sensitive to the allergen but does not cause symptoms. When sensitized people subsequently encounter the allergen, the basophils and mast cells with IgE on their surface release substances (such as histamine, prostaglandins, and leukotrienes) that cause swelling or inflammation in the surrounding tissues. Such substances begin a cascade of reactions that continue to irritate and harm tissues. These reactions range from mild to severe.
Most allergic reactions are mild, consisting of watery and itchy eyes, a runny nose, itchy skin, and some sneezing. Rashes (including hives) are common and often itch. Hives, also called urticaria, are small, red, slightly elevated areas of swelling (wheals) that often have a pale center. Swelling may occur in larger areas under the skin (called angioedema—see Angioedema). Swelling is caused by fluids leaking from blood vessels. Depending on which areas of the body are affected, angioedema may be serious. Allergies may trigger attacks of asthma.
Certain allergic reactions, called anaphylactic reactions (see see Anaphylactic Reactions), can be life threatening. The airways can narrow (constrict), causing wheezing, and the lining of the throat and airways may swell, interfering with breathing. Blood vessels can widen (dilate), causing a dangerous fall in blood pressure.
Doctors first determine whether a reaction is allergic. They may ask whether the person has close relatives with allergies because a reaction is more likely to be allergic in such cases. Blood tests are usually done to detect a type of white blood cell called eosinophils. Eosinophils, although present in everyone are usually produced in greater numbers when an allergic reaction occurs.
Because each allergic reaction is triggered by a specific allergen, the main goal of diagnosis is to identify that allergen. Often, the person and doctor can identify the allergen based on when the allergy started and when and how often the reaction occurs (for example, during certain seasons or after eating certain foods).
Skin tests (see Skin Tests) are the most useful way to identify specific allergens. Usually, a skin prick test is done first. Dilute solutions are made from extracts of pollens (of trees, grasses, weeds, or fungal spores), dust, animal dander, insect venom, foods, and some drugs. A drop of each solution is placed on the person's skin, which is then pricked with a needle. If the person is allergic to one or more of these substances, the person has a wheal and flare reaction, indicated by the following:
The skin prick test can identify most allergens. If no allergen is identified, a tiny amount of each solution can be injected into the person's skin (intradermal test). This type of skin test is more likely to detect a reaction to an allergen.
Before skin tests are done, people are asked to stop taking antihistamines and certain antidepressants called tricyclic antidepressants (such as amitriptyline) and monoamine oxidase inhibitors (such as selegiline). These drugs may suppress a reaction to the tests. Some doctors also do not test people who are taking beta-blockers because if such people have an allergic reaction, the consequences are more likely to be serious. In addition, beta-blockers may interfere with the drugs used to treat serious allergic reactions.
The radioallergosorbent test (RAST) is used when skin tests cannot be used—for example, when a rash is widespread. This test measures blood levels of different types of IgE. The immune system produces different types of IgE in response to each allergen. If a large amount of one type of IgE is detected, the immune system is mounting an allergic reaction against that allergen. Thus, this test helps doctors identify the allergen.
However, skin tests and RAST may not detect all allergies, and they sometimes indicate that people are allergic to an allergen when they are not (called a false-positive result).
Avoiding an allergen, if possible, is the best approach. Avoiding an allergen may involve the following:
Because some allergens, especially airborne allergens, cannot be avoided, allergen immunotherapy (also called desensitization), usually allergy shots or injections, can be given to desensitize people to the allergen. With allergen immunotherapy, allergic reactions can be prevented or reduced in number and/or severity. However, allergen immunotherapy is not always effective. Some people and some allergies tend to respond better than others.
Immunotherapy is used most often for allergies to pollens, house dust mites, molds, and venom of stinging insects. When people are allergic to unavoidable allergens, such as insect venom, immunotherapy helps prevent anaphylactic reactions (see see Anaphylactic Reactions). Sometimes it is used for allergies to animal dander, but such treatment is unlikely to be useful. Immunotherapy for food allergies is usually not advised because it can cause severe reactions and is less effective. Also, foods can usually be avoided.
Immunotherapy is not used when the allergen, such as penicillin and other drugs, can be avoided. However, if people need to take a drug that they are allergic to, immunotherapy, closely monitored by a doctor, can be done to desensitize them.
In immunotherapy, tiny amounts of the allergen are usually injected under the skin. The dose is gradually increased until a dose adequate to control symptoms (maintenance dose) is reached. A gradual increase is necessary because exposure to a high dose of the allergen too soon can cause an allergic reaction. Injections are usually given once or twice a week until the maintenance dose is reached. Then injections are usually given every 2 to 6 weeks. The procedure is most effective when maintenance injections are continued throughout the year, even for seasonal allergies.
Alternatively, high doses of the allergen may be placed under the tongue (sublingual) and held there for a few minutes, then swallowed. The dose is gradually increased, as for injections. The sublingual technique is relatively new, and how often the dose should be given has not been established. It ranges from every day to 3 times a week.
Allergen immunotherapy may take 3 to 4 years to complete.
Because immunotherapy injections occasionally cause dangerous allergic reactions, people remain in the doctor's office for at least 30 minutes afterward. If they have mild reactions to immunotherapy (such as sneezing, coughing, flushing, tingling sensations, itching, chest tightness, wheezing, and hives), a drug—usually an antihistamine, such as diphenhydramine or loratadine—may help. For more severe reactions, epinephrine (adrenaline) is injected.
Avoiding the allergen is the best way to treat as well as prevent allergies. If mild symptoms occur, antihistamines are often all that is needed. If they are ineffective, other drugs, such as mast cell stabilizers and corticosteroids may help. Nonsteroidal anti-inflammatory drugs (NSAIDs) are not useful. Severe symptoms, such as those involving the airways (including anaphylactic reactions), require emergency treatment.
Whenever possible, pregnant women with allergies should avoid allergens in order to control their symptoms. If symptoms are severe, pregnant women should use an antihistamine nasal spray. They should take antihistamines by mouth (oral antihistamines) only if antihistamine nasal sprays do not provide adequate relief. Women who are breastfeeding should also try to avoid antihistamines, but if antihistamines are necessary, antihistamine nasal sprays are preferred to oral antihistamines. If oral antihistamines are essential for controlling symptoms, they should be taken immediately after feeding the baby.
The drugs most commonly used to relieve the symptoms of allergies are antihistamines. Antihistamines block the effects of histamine (which triggers symptoms) rather than stop its production. Taking antihistamines partially relieves the runny nose, watery eyes, and itching and reduces the swelling due to hives or mild angioedema. But antihistamines do not ease breathing when airways are constricted. Some antihistamines are also mast cell stabilizers (see Mast cell stabilizers).
Antihistamines are available as tablets, capsules, or liquid solutions to be taken by mouth or as nasal sprays, eye drops, or lotions or creams. Which is used depends on the type of allergic reaction. Some antihistamines are available without a prescription (over-the-counter), and some require a prescription. Some that used to require a prescription are now available over the counter (OTC).
Diphenhydramine is available OTC as a lotion, cream, gel, or spray that can be applied to the skin to relieve itching, but it should not be used. Its effectiveness is unproved, and it can cause allergic reactions (such as a rash). Also, it can cause extreme drowsiness in children who are also taking an antihistamine by mouth.
Antihistamines have anticholinergic effects, such as drowsiness, dry mouth, blurred vision, constipation, difficulty with urination, confusion, and light-headedness (particularly after a person stands up), as well as drowsiness. Often, prescription antihistamines have fewer of these effects.
Some antihistamines are more likely to cause drowsiness (sedation) than others. Antihistamines that cause drowsiness are widely available OTC. People should not take these antihistamines if they are going to drive, operate heavy equipment, or do other activities that require alertness. These antihistamines should not be given to children under 2 years old because they may have serious or life-threatening side effects. These antihistamines are also a particular problem for older people (see see Sidebar 1: Anticholinergic: What Does It Mean?) and for people with glaucoma, benign prostatic hyperplasia, constipation, or dementia because of the drugs' anticholinergic effects. In general, doctors use antihistamines cautiously in people with cardiovascular disease.
Not everyone reacts the same way to antihistamines. For example, Asians seem to be less susceptible to the sedative effects of diphenhydramine than are people of Western European origin. Also, antihistamines cause the opposite (paradoxical) reaction in some people, making them feel nervous, restless, and agitated.
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Mast cell stabilizers:
Mast cell stabilizers inhibit mast cells from releasing histamines and other substances that cause swelling and inflammation. Mast cell stabilizers are taken when antihistamines and other drugs are not effective or have bothersome side effects. These drugs may help control allergic symptoms. These drugs include azelastine, cromolyn, iodoxamide, ketotifen, nedocromil, olopatadine, and pemirolast. Azelastine, ketotifen, and olopatadine are also antihistamines.
Cromolyn is available by prescription for use with an inhaler or nebulizer (which delivers the drug in aerosol form to the lungs), as eye drops, or in forms to be taken by mouth. It is available without a prescription as a nasal spray. Cromolyn usually affects only the areas where it is applied, such as the back of the throat, lungs, eyes, or nose. When taken by mouth, cromolyn can relieve the digestive symptoms of mastocytosis (see see Mastocytosis), but it is not absorbed into the bloodstream and thus has no effect on other allergy symptoms.
When antihistamines and mast cell stabilizers cannot control allergy symptoms, a corticosteroid may help. Corticosteroids can be taken as a nasal spray to treat nasal symptoms or through an inhaler, usually to treat asthma.
Doctors prescribe a corticosteroid (such as prednisone) to be taken by mouth only when symptoms are very severe or widespread and all other treatments are ineffective. If taken by mouth at high doses and for a long time (for example, for more than 3 to 4 weeks), corticosteroids can have many, sometimes serious side effects (see see Sidebar 3: Corticosteroids: Uses and Side Effects). Therefore, corticosteroids taken by mouth are used for as short a time as possible.
Creams and ointments that contain corticosteroids can help relieve the itching associated with allergic rashes. One corticosteroid, hydrocortisone, is available OTC.
Leukotriene modifiers, such as montelukast, are anti-inflammatory drugs used to treat mild persistent asthma and seasonal allergic rhinitis. They inhibit leukotrienes, which are released by some white blood cells and mast cells when they are exposed to an allergen. Leukotrienes contribute to inflammation and cause airways to constrict.
Omalizumab is a monoclonal antibody (which is a manufactured [synthetic] antibody designed to interact with a specific substance). Omalizumab binds to IgE, an antibody that is produced in large amounts during an allergic reaction, and prevents IgE from binding to mast cells and basophils and triggering an allergic reaction. Omalizumab may be used to treat allergic rhinitis or persistent or severe asthma when other treatments are ineffective. When it is used, the dose of a corticosteroid can be reduced. It is given by injection under the skin (subcutaneously).
Severe allergic reactions, such as an anaphylactic reaction, require prompt emergency treatment. People who have severe allergic reactions should always carry a self-injecting syringe of epinephrine. Many of these people also carry antihistamine pills. If a severe reaction occurs, these treatments should be used as quickly as possible. Usually, the combination of epinephrine and an antihistamine stops the reaction, at least temporarily. Nonetheless, people who have had a severe allergic reaction should go to the hospital emergency department, where they can be closely monitored and treatment can be repeated or adjusted as needed.
Last full review/revision August 2012 by Peter J. Delves, PhD