Introduction
Allergic reactions (hypersensitivity reactions) are inappropriate immune responses 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 to certain antigens (called allergens), 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, are inhaled, are eaten, or are injected. An allergic reaction can occur as part of a seasonal allergy (such as hay fever), caused by exposure to such substances as grass or ragweed pollen. Or an allergic reaction can be triggered by taking a drug, eating certain foods, or breathing in dust or animal dander.
In most 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 a person sensitive to the allergen but does not cause symptoms. When the sensitized person subsequently encounters the allergen, the cells that have 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.
Symptoms and Diagnosis
Most allergic reactions are mild, consisting of watery, itchy eyes, a runny nose, itchy skin, and some sneezing. Rashes (including hives) are common and often itch. Swelling may occur in small areas of the skin (with hives) or in larger areas under the skin (as angioedema (see Section 16, Chapter 185)). 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 Section 16, Chapter 185), can be life threatening. The airways can constrict (causing wheezing), and blood vessels can dilate (causing a 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 performed to detect a type of white blood cell called eosinophils. Eosinophils are produced in large numbers as a result of an allergic reaction.
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 are the most useful way to identify specific allergens. Usually, a skin prick test is performed first. Dilute solutions are made from extracts of pollens (from 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: A pale, slightly elevated swelling--the wheal--appears at the pinprick site within 15 to 20 minutes. The wheal is surrounded by a well-defined area of redness--the flare. The resulting area is about ½ inch in diameter. 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. This type of skin test is more likely than the skin prick test to detect a reaction to an allergen. Antihistamines should not be taken before skin tests, because they may suppress a reaction to the tests.
The radioallergosorbent test (RAST) is used when skin tests cannot be used--for example, when a skin rash is widespread. This test measures blood levels of different types of IgE that are specific to particular allergens and thus helps doctors identify the allergen.
Prevention
Avoiding an allergen, if possible, is the best approach. Avoiding an allergen may involve discontinuing a drug, keeping a pet out of the house, installing high-efficiency air filters, or not eating a particular food. A person with severe seasonal allergies may consider moving to an area that does not have the allergen. A person with an allergy to house dust should remove items that collect dust.
Allergen Immunotherapy: Because some allergens, especially airborne allergens, cannot be avoided, allergen immunotherapy, commonly called allergy shots or injections, can be given to desensitize a person to the allergen. With allergen immunotherapy, allergic reactions can be prevented or reduced in number 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, insect venoms, and animal dander. Immunotherapy for food allergies is usually not advised because it can cause severe reactions and is less effective. Also, foods can usually be avoided.
In immunotherapy, tiny amounts of the allergen are 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 produce 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 rhinitis. Allergen immunotherapy may take 3 to 4 years to complete.
Because immunotherapy injections occasionally cause dangerous allergic reactions, the person remains in the doctor's office for at least 20 minutes afterward. If the person has 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.
Allergen immunotherapy may be used to prevent anaphylactic reactions (see Section 16, Chapter 185) in people who are allergic to unavoidable allergens, such as insect stings. Immunotherapy is not used when the allergen, such as penicillin and other drugs, can be avoided. However, for people who need to take a drug they are allergic to, immunotherapy, closely monitored by a doctor, can be rapidly performed to desensitize them.
Treatment
Antihistamines: The drugs most commonly used to relieve the symptoms of allergies are antihistamines. Some antihistamines are available without a prescription, and some require a prescription. Nonprescription (over-the-counter) antihistamines are a particular problem for older people (see Section 2, Chapter 18). Often, prescription antihistamines have fewer side effects (such as sleepiness, dry mouth, blurred vision, constipation, and difficulty with urination). Antihistamines block the effects of histamine rather than stop its production. Taking antihistamines partially relieves the itching and reduces the swelling due to hives or mild angioedema.
 See the drug table Some Antihistamines.
Cromolyn: Cromolyn may help control allergic symptoms. It is available by prescription for use with an inhaler or nebulizer (which delivers the drug to the lungs) or as eye drops. 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 is not absorbed into the bloodstream, but it can relieve the digestive symptoms of mastocytosis. Cromolyn inhibits mast cells from releasing substances that damage nearby tissues.
Corticosteroids: When antihistamines and cromolyn 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. If symptoms are very severe or widespread, taking a corticosteroid (such as prednisone) by mouth may be necessary. If taken by mouth for more than 3 to 4 weeks, corticosteroids have many, sometimes serious side effects (see Section 5, Chapter 67). Therefore, corticosteroids taken by mouth are prescribed only for severe symptoms when all other treatments are ineffective, and they are given for as short a time as possible.
Emergency Treatment: 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. Such people often also carry antihistamine tablets, which are also taken as quickly as possible. Usually, the combination of epinephrine and an antihistamine stops the reaction. 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.
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