(an″ă-fĭ-lak′sĭs )

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[ana- + (pro)phylaxis]
A sudden, severe allergic reaction between an allergenic antigen and immunoglobulin E (IgE) bound to mast cells, which stimulates the sudden release of immunological mediators locally or throughout the body. The first symptoms occur within minutes, and a recurrence may follow hours later (late-stage response). Anaphylaxis can only occur in someone previously sensitized to an allergen because the initial exposure causes immunoglobulin E (IgE) to bind to mast cells. Anaphylaxis may be local or systemic. Local anaphylactic reactions include hay fever, hives, and allergic gastroenteritis. Systemic anaphylaxis produces peripheral vasodilation, bronchospasm, and laryngeal edema and can be life-threatening.
anaphylactic (an″ă-fĭ-lak′tik)
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, adj.

IgE antibodies react when the allergen is introduced a second time. The mast cells release packets containing chemical mediators (degranulators) that attract neutrophils and eosinophils and stimulate urticaria (hives) and vasodilation, increased vascular permeability, and smooth muscle spasm, esp. in the bronchi and gastrointestinal tract. Chemical anaphylactic mediators include histamine, proteases, chemotactic factors, leukotrienes, prostaglandin D, and cytokines, e.g., TNF-α and interleukins 1, 3, 4, 5, and 6. The most common agents triggering anaphylaxis are food, drugs, and insect stings. Local anaphylactic reactions are also commonly triggered by pollens, e.g., hay fever, allergic rhinitis, allergic asthma.
SEE: anaphylactic shock

Local anaphylaxis causes such signs as urticaria, edema, warmth, and erythema to appear at the site of allergen-antibody interaction. In systemic anaphylaxis the respiratory tract, cardiovascular system, skin, and gastrointestinal system are involved. The primary signs are urticaria, angioedema, flushing, wheezing, dyspnea, increased mucus production, nausea and vomiting, and feelings of generalized anxiety. Systemic anaphylaxis may be mild or severe enough to cause shock when massive vasodilation is present.

Treatment for systemic anaphylaxis includes immediate administration of epinephrine intramuscularly (usually in the vastus lateralis muscle of the thigh). A second dose may be needed in about 15% of patients. Additional therapies include protection of the airway and administration of oxygen; antihistamines, e.g., diphenhydramine or cimetidine to block histamine H1 and H2 receptors; IV fluids to support blood pressure; and vasopressors, to prevent or treat shock. Epinephrine is also used to treat bronchospasm. Epinephrine may also be given intravenously or, in mild cases, subcutaneously. Corticosteroids may be used to prevent recurrence of bronchospasm and increased vascular permeability.

Prevention: A history of allergic reactions, particularly to drugs, blood, or contrast media, is obtained. The susceptible patient is observed for reaction during and immediately after administration of any of these agents. The patient is taught to identify and avoid common allergens and to recognize an allergic reaction.

Patients also should be taught to always wear tags identifying allergies to medications in order to prevent inappropriate treatment during an emergency. Those who have had an anaphylactic reaction and are unable to avoid future exposure to allergens should carry a kit containing a syringe of epinephrine and be taught how to administer it. Patients allergic to the venom of Hymenoptera (bees, wasps, hornets) can receive desensitization.

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