Allergy is an abnormal bodily reaction to a particular substance. An otherwise harmless substance may cause mild to extremely severe symptoms in a person who is allergic to it. Such symptoms range from sneezing and breathing difficulty to irritation of the skin, eyes, and stomach. Few people die from allergies. But allergies can significantly reduce quality of life.
Allergies rank mong the most common causes of chronic (ongoing) medical disease in the United States and other developed countries. The most common allergic diseases include allergic rhinitis, known as nasal allergies or hay fever, and allergic conjunctivitis, an irritation of the eye. Physicians consider asthma to be an allergic disease of the respiratory system. Other disorders commonly associated with allergies include digestive disturbances, headaches, itchy skin patches called hives, and eczema, a skin disorder marked by itching and inflammation. The incidence of most allergic diseases has increased steadily in the United States and many other countries. Physicians who treat allergies are called allergists.
An otherwise harmless substance that causes an allergy is called an allergen. A relatively small number of allergens account for the majority of allergies in people. The most common allergens found indoors include dust mites, mold spores, and pet dander (bits of dried skin and hair or feathers). Outdoor allergens include many substances in the air we breathe, such as pollen from grass, weeds, and trees, as well as mold spores. The most common food allergens for children are milk, eggs, peanuts, soybeans, wheat, nuts that grow on trees, and fish. In adults, common food allergens include peanuts, tree nuts, fish, and shellfish. Medications can also cause allergies. Certain antibiotic drugs, especially antibiotics in the penicillin family, are a common cause of medication allergies. The venom of such stinging insects as bees, wasps, hornets, and fire ants can also cause allergies.
Allergens may enter the body through several different routes. For example, food and medicine allergens enter through the mouth. Inhaled allergens—such as dust mites, plant pollens, molds spores, and animal dander—are breathed in through the nose and mouth and into the respiratory system. Other allergens may enter through the skin, either by contact with its surface or through an injection or sting.
Allergies are reactions of the immune system. One of the major functions of the immune system is to detect and respond to foreign substances that enter the body. If a foreign substance is a harmful one, such as a bacterium or virus, the immune system attacks to counter the threat. However, some substances, such as food, are needed by the body, and other substances, such as dust, are harmless. These substances are typically ignored or tolerated by the immune system. An allergy develops when the immune system attacks rather than tolerates a harmless substance.
Allergic reactions.
There are many different types of allergic reaction. The most common type is called the type I allergic reaction or IgE-mediated allergic reaction. IgE stands for immunoglobulin E. IgE is an antibody produced by the immune system. Antibodies are proteins that protect the body from infection and from toxins (poisons) secreted by some bacteria.
The immune system produces a different form of IgE for every allergen it encounters. Some IgE circulates in the blood, but most IgE is bound to the surface of mast cells. Mast cells are protective cells found in almost every organ of the body.
Allergens that enter the body are taken up by nearby immune cells and eventually transported to mast cells. The allergen binds to the corresponding IgE on the mast cell’s surface. The binding sets off a series of events. First, mast cells release a chemical called histamine. Histamine, in turn, triggers many of the symptoms of allergic reactions, such as sneezing, itching, and swelling. Next, mast cells produce other chemicals called mediators. Mediators make an allergic reaction last longer. They do this by directly causing allergic symptoms and by attracting other immune cells that prolong and intensify the reaction.
Allergic reactions can occur in many different organs of the human body. Scientists do not know why the same allergen entering in the same way will cause a skin rash in one person, breathing difficulty in another person, and intestinal problems in still another person. They do know that the immune cells involved are attracted to different parts of the body by signals from both the immune system and the target organs. This process is called cell trafficking. However, scientists know little about how cell trafficking actually works.
Occasionally, an allergen will trigger a sudden and overwhelming reaction that affects several parts of the body at the same time. This reaction is called anaphylaxis << an uh fuh LAK sihs >> . Symptoms of anaphylaxis can include the swelling and closing of the throat, spasms of the airways in the lungs, and a drop in blood pressure. If severe, anaphylaxis can lead to shock and even death. Many allergies can lead to anaphylaxis, but the condition most commonly results from allergies to certain foods, medicines, and insect stings.
The development of allergies.
Scientists know little about why people develop allergies. But they do know that the development of allergies is influenced by both genetic (hereditary) and environmental factors.
Scientists know that atopy, the tendency to develop allergies, runs in families. They have identified many different genes that play a role in the development of allergies. These genes include those that control the production of IgE and IgE receptors, proteins on the surface of cells to which IgE binds. The genes involved also include genes that govern production of mediators. Despite being able to identify these genes, scientists cannot predict who will develop allergies.
The risk of developing allergies can increase with exposure to many substances in the environment. Exposure to cigarette smoke, for example, ranks as a strong risk factor for allergies. Common infections, especially respiratory infections, can increase a child’s chance of developing allergies. People who live in homes with pets, excessive dust, or mold are exposed to potential allergens, which can increase the risk of developing allergies.
However, scientists believe that some exposure to certain substances in the environment may actually decrease the risk of allergies. Similarly, they think that a lack of exposure to certain substances may increase the risk of developing allergies. Scientists call this idea the hygiene hypothesis. According to the hygiene hypothesis, children who are exposed to relatively high levels of bacteria products, such as those who grow up on farms, may be protected against developing allergies. The hypothesis may explain why allergic diseases are more prevalent in urban areas than rural areas. It also may explain why scientists have seen an increase in allergies in many regions as overall sanitation has improved.
Diagnosis.
Often, observation can reveal the cause of an allergic reaction. In many cases, however, the cause of the reaction is not apparent. Doctors often diagnose such allergies using skin testing. In this procedure, a health care professional places extracts of purified allergens as drops on the skin. A small pointed device is used to push the liquid into the skin. For each substance to which the person is allergic, a small bump surrounded by a ring of red skin will appear within minutes.
Blood tests can also be used to diagnose allergies. Such tests measure concentrations of specific forms of IgE. For example, a person allergic to ragweed will have a form of IgE in the blood that binds only to ragweed pollen. The allergy’s severity is related to the concentration of the IgE in the blood.
Treatment.
The most effective way to treat an allergic disease is to avoid exposure to the allergens that trigger it. Certain allergens, such as foods and pets, can be avoided entirely. However, it is usually impossible to totally eliminate indoor allergens. The only way to completely avoid outdoor allergens is to stay indoors, which is often impossible.
Common drugs used to relieve allergy symptoms include antihistamines. These drugs block histamine from binding to histamine receptors on the body’s cells. Doctors also treat allergy symptoms using drugs that fight inflammation. Anti-inflammatory drugs called glucocorticoids block the action of mediators. In the treatment of allergies, such drugs are often referred to simply as steroids. Some allergy drugs are swallowed as tablets. Others include skin creams and ointments, eye drops, nasal sprays, and inhaled sprays. Still other allergy medications can be injected. Such drugs are usually used to treat emergency cases of anaphylaxis.
Doctors also treat allergies using desensitization or specific allergen immunotherapy. In this process, the doctor combines concentrated extracts of a patient’s known allergens to make a vaccine for that patient. At first, an extremely dilute solution is injected. The concentration of allergen is gradually increased. The patient becomes less sensitive to the allergens as the course of shots reaches the highest concentration. Additional shots are then given every one to four weeks for several years to maintain the effect. After treatment, many people no longer suffer reactions when exposed to allergens.
