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Dermatology => Skin => Allergy
Allergy
INTRODUCTION
Allergy, exaggerated and sometimes harmful reactions to external substances, called allergens. Allergy may result from exposure to such common allergens as plant pollens from grasses, trees, or ragweed; animal danders, which are tiny scales shed from the skin and hair of cats and other furred animals; insects, such as house dust mites, bees, and wasps; and drugs, such as penicillin. The most common food allergies are caused by eggs, milk, peanuts, shellfish, wheat, and chocolate. Researchers estimate that nearly 50 million people in the United States suffer from allergies-about 20 percent of the population.
In an allergic reaction, the immune system mistakenly interprets a harmless substance as a harmful one. The immune system responds by producing antibodies called immunoglobulin E (IgE). These IgE antibodies are designed to help neutralize specific parasitic invaders and protect the person from future exposures. Upon first exposure to an allergen, no allergic symptoms develop. But when the person is exposed to the same substance at a later time, the IgE antibodies against the allergen activate an allergic reaction. Allergy symptoms may include itching, sneezing, a stuffy nose, watery eyes, inflammation of the airways in the lungs and wheezing (known as asthma), and even allergic shock and death in rare situations.
HOW ALLERGIES DEVELOP
After allergy antibodies have been formed in a person's body in response to a particular allergen, an allergic reaction can occur when the person comes in contact with that allergen. Depending on the substance, allergens can be inhaled, eaten, injected, or contacted by the skin. When allergy antibodies are activated by an allergen, they cause body cells to release a substance called histamine, a chemical that dilates blood vessels, promotes fluid secretions, and stimulates nerves that cause muscles to spasm. These reactions create various allergy symptoms.
In a person with hay fever, for example, pollen allergens cause a number of symptoms. When pollen is breathed in through the nose, the release of histamine in the nasal passages causes violent, repetitive sneezing, release of watery fluids, and itching. Sometimes pollen triggers tissue swelling, which can cause blocked nasal passages, with consequent loss of smell and taste. When pollen allergens affect the eyes, they become itchy, red, and watery. Allergens that affect the lungs cause secretion of mucus and inflammation, swelling, and narrowing of the airways, resulting in asthma. Symptoms include spasms of the airways and sudden difficulty in breathing.
Allergic reactions can also be triggered throughout the entire body, rather than in one specific location. Called allergic or anaphylactic shock, this response occurs when many cells throughout the body react simultaneously to an allergen, such as bee sting venom. The person may experience hives or welts on the skin, itching all over the body, asthmatic spasms in the lungs, or a sudden drop in blood pressure. An additional danger is the possibility of swelling in the throat, tongue, and larynx (voice box), which can close the upper airways and cause fatal choking.
Food allergies are fairly common, but they are poorly understood. When foods are digested and the nutrients are absorbed in the intestine, substances in the food probably stimulate the allergic response. The reaction can occur in the intestine itself, resulting in cramps or diarrhea; in the skin in the form of eczema; or all over the body, also causing allergic shock. Food allergy, which is an immune response, is often confused with food intolerance, which may cause similar symptoms of gastrointestinal discomfort. Food intolerance, however, is caused by many factors other than an allergen; a person may lack an enzyme to digest a particular food, for instance. The resulting symptoms of food intolerance are not triggered by an immune response.
Researchers have identified a definite genetic predisposition to allergies. For example, if one parent has allergies, there is an increased risk that some of the children will also have allergies, although the children may not be sensitive to the same allergens as the parent. If both parents have allergies, the risk that the children will develop allergies is even greater. The most typical time for allergies to develop is in young adulthood, although allergies can develop in a person of almost any age, even within a few months after birth. Allergies in infants are most commonly associated with foods and viral respiratory infections. For reasons that are not clearly understood, children with allergies tend to outgrow them. The child's body somehow readjusts its response to allergens, even those that cause severe reactions, such as food, drugs (especially penicillin), and stinging insects.
Patients are sometimes surprised by the abrupt onset of allergies in adult life. This can occur when the combination of a person's genetic makeup and a longstanding but unnoticed reactivity to an allergen finally culminates in a detectable disease. This so-called allergic threshold is crossed when allergens finally produce enough reaction in the body to cause detectable symptoms. Several as yet unknown genes may be involved in this process.
Many people think that emotions, such as stress, cause allergies, but most doctors feel this is incorrect. In fact, the opposite may be true: People with allergies live with symptoms that can produce serious emotional upset. For example, a person with asthma may have fears about losing the ability to breathe, or possibly choking to death. Allergies, including asthma, are caused by biological factors, although emotions may aggravate an allergic reaction.
DIAGNOSIS AND TREATMENT
Diagnosing and treating allergies is usually performed by an allergist, a physician trained to understand the body's immune response. When visiting an allergist for the first time, a patient is usually asked which substances seem to provoke symptoms and whether other family members have allergies. The doctor conducts a brief physical examination, looking in the nose, eyes, ears, and throat, listening to the chest, and examining the skin.
In many cases the allergist will perform allergy skin tests. These are painless injections or scratches into the surface of the skin with tiny amounts of specific allergens, such as pollens or house dust mites. The injection produces a tiny bump in the skin, no larger than the head of a pin. When a person has a positive skin test response (usually occurring after 15 to 30 minutes), there is a local reaction in the skin. The tiny bump gets slightly bigger, slightly itchy, and is surrounded by some redness caused by dilating blood vessels in the area. This reaction indicates to the allergist that cells in the skin contain specific antibodies to the allergens used in the injection. This reaction provides a quick technique for testing and diagnosing specific allergies. Sometimes blood samples evaluated in a laboratory are used to measure the blood levels of IgE antibodies specific to individual allergens, although skin tests have proven to be more accurate, faster, and less expensive in allergy diagnosis.
For most patients with allergies, medicines are used to begin therapy. Most forms of hay fever are easily managed with antihistamines, which relieve the symptoms, such as itching and sneezing, produced by histamine. Asthma is usually treated with medications taken orally or inhaled in vapor form using a metered-dose inhaler. Asthma medications include bronchodilators (drugs that expand the air passages) and anti-inflammatory steroids, which suppress the immune response that causes airway inflammation. In cases of anaphylactic shock, emergency treatment with an injection of adrenaline, also known as epinephrine, is required. This injection quickly widens blood vessels and opens up constricted airways.
If medicines cannot control allergy symptoms, the allergist may suggest allergen immunotherapy, a series of injections of the offending allergens. These injections, or allergy shots, help desensitize the patient to the allergens, thereby reducing the allergic response. For safety reasons, the injections begin with very small allergen doses, close to the amount used in a skin test. The amount of the allergen injected is increased each week for many weeks, until high doses of injected allergens are reached. The goal of immunotherapy is to produce blocking or neutralizing antibodies that provide a protective response in the cells, preventing allergens from binding to the allergic antibodies. When this blocking is achieved, little or no histamine is released in response to the allergens, and allergic symptoms are reduced or eliminated. Allergen immunotherapy is especially effective in overcoming stinging insect allergies, which, if left untreated, can result in a fatal allergic reaction to an insect bite. Immunotherapy is also effective for severe hay fever and, in some patients, for severe, chronic allergic asthma.
In many cases the best allergy treatment is the removal, if possible, of the offending allergens from the patient's environment. For example, the best way to deal effectively with an allergy to cats is to remove cats from the patient's surroundings, although desensitizing injections containing cat extracts are being tested. Unfortunately, some allergens, such as plant pollens, are impossible to avoid, since they float freely through the air. Contact with pollens can be reduced by keeping windows closed and using air conditioners to filter and cool indoor air. House dust mites, a common allergen, can be minimized by frequent cleaning with safe chemicals.
Food allergies are more difficult to diagnose and treat than other types of allergies. Skin tests are unreliable, and blood tests can be inconclusive. When a particular food is suspect, the patient simply should not eat it. If the culprit food is unknown, the allergist may put the patient on a special diet that eliminates various foods. If symptoms decline, the allergist will reintroduce each of the foods one at a time to help identify which food is the offending allergen.
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