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[allo- + Gr. -ergia, work, activity]
An immune response to a foreign antigen that results in inflammation and organ dysfunction. Allergies range from annoying to life-threatening. They include systemic anaphylaxis, urticaria, eczematous dermatitis, hay fever, and rhinitis. They affect about 20% of Americans and can be triggered by inhalation (pollen, dust mites), direct contact (poison ivy), ingestion (drugs, foods), or injection (stinging insects, drugs). Allergic responses may be initiated and sustained by occupational exposure to allergens, and by foods, animals, fungal spores, metals, and rubber products. The most severe cases are often associated with Hymenoptera stings, penicillin products, radiological contrast media, and latex.
SYN: SEE: hypersensitivity reaction
SEE: allergen; atopy

The immune system has two main functions: first, to identify germs and parasites that may harm the body; and second, to use toxic defenses against attacks by these organisms. Allergic reactions occur when immune functions are turned on by an agent richly endowed with alien antigens. Once the immune system has been sensitized, subsequent exposure results in the binding of specific immunoglobulins (esp. IgE) or the activation of immunologically active cells, e.g., mast cells, basophils, or T cells. These can release inflammatory chemicals (histamines, kinins, interleukins) that create allergic symptoms.

Nasal inflammation, mucus production, watery eyes, itching, rashes, tissue swelling, bronchospasm, stridor, and shock are all symptoms of allergy.

A history of exposure and reaction is crucial to the diagnosis of allergy. Tests for specific allergies include skin prick tests, intradermal injections, or blood tests (measurements of antigen-specific immunoglobulins).

Avoiding allergens is the first step in treatment. Effective drugs for allergic symptoms include antihistamines, corticosteroids, and epinephrine, depending on the severity of the reaction. Antigen desensitization (immunotherapy) may be used by experienced professionals, but this technique may occasionally trigger severe systemic reactions.

Before any drug is given, the health care provider should determine if the patient has a history of allergy. Patients receiving injected drugs or blood products are closely observed for rash, itch, wheezing, or hypotension. If an allergic reaction begins, medications are prescribed for immediate management. Patients are taught to identify and avoid common allergens and to recognize an allergic reaction. The use of drugs for the chronic management of allergies is explained, and the patient is advised about potential adverse effects. If a patient needs injectable epinephrine for emergency outpatient treatment of anaphylaxis, both the patient and family are instructed in its use.

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