An allergy is a hypersensitivity disorder of the immune system. Symptoms include red eyes, itchiness, and runny nose, eczema, hives, or an asthma attack. Allergies can play a major role in conditions such as asthma. In some people, severe allergies to environmental or dietary allergens or to medication may result in life-threatening reactions called anaphylaxis. Food allergies and reactions to the venom of stinging insects such as wasps and bees are more often associated with these severe reactions. Not all reactions or intolerances are forms of allergy. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid. Allergy is one of four forms of hypersensitivity and is formally called type I (or immediate) hypersensitivity. Allergic reactions are distinctive because of excessive activation of certain white blood cells called mast cells and basophils by a type of antibody called immunoglobulin E (IgE). This reaction results in an inflammatory response which can range from uncomfortable to dangerous. A variety of tests exist to diagnose allergic conditions. If done they should be ordered and interpreted in light of a person's history of exposure as many positive test results do not mean a clinically significant allergy. Tests include placing possible allergens on the skin and looking for a reaction such as swelling and blood tests to look for an allergen-specific IgE. Treatments for allergies include avoiding known allergens, steroids that modify the immune system in general, and medications such as antihistamines and decongestants which reduce symptoms. Many of these medications are taken by mouth, although epinephrine, which is used to treat anaphylactic reactions, is injected. Immunotherapy uses injected allergens to desensitize the body's response. Mild allergies like hay fever are very common.

Signs and symptoms

Many allergens such as dust or pollen are airborne particles. In these cases, symptoms arise in areas in contact with air, such as eyes, nose, and lungs. For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, itching, and redness of the eyes. Inhaled allergens can also lead to asthmatic symptoms, caused by narrowing of the airways (bronchoconstriction) and increased production of mucus in the lungs, shortness of breath (dyspnea), coughing and wheezing. Aside from these ambient allergens, allergic reactions can result from foods, insect stings, and reactions to medications like aspirin and antibiotics such as penicillin. Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhea, itchy skin, and swelling of the skin during hives. Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis. Insect stings, antibiotics, and certain medicines produce a systemic allergic response that is also called anaphylaxis; multiple organ systems can be affected, including the digestive system, the respiratory system, and the circulatory system. Depending on the rate of severity, it can cause cutaneous reactions, bronchoconstriction, edema, hypotension, coma, and even death. This type of reaction can be triggered suddenly, or the onset can be delayed. The severity of this type of allergic response often requires injections of epinephrine, sometimes...

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