anaphylaxis, hypersensitive state that may develop after introduction of a foreign protein or other antigen into the body tissues. When an anaphylactic state exists, a second dose of the same protein (commonly an antibiotic such as penicillin, or certain insect venoms) will cause a violent allergic reaction. Anaphylaxis results from the production of specific antibodies in the tissues in very high concentration; the violent reaction is produced by the neutralization of antigens by the antibodies. The histamines released during the reaction are thought to cause the most damage, i.e., severe vasodilation and loss of capillary fluid, resulting in circulatory collapse. Other symptoms include urticaria or edema, choking, coughing, shock, and loss of consciousness. Death may occur within 5 to 10 min if no medical help is available. Anaphylaxis differs from immunity; in immunity, antibodies circulate in the blood and neutralize antigens without producing a violent reaction. See also allergy; serum sickness.

Severe, immediate, potentially fatal bodily reaction to contact with a substance (antigen) to which the individual has previously been exposed. Often triggered by antiserum, antibiotics, or insect stings, the reaction's symptoms include skin flushing, bronchial swelling (with difficulty breathing), and loss of consciousness. Shock may follow. Milder cases may involve hives and severe headache. Treatment, consisting of injection of epinephrine, followed by antihistamines, cortisone, or similar drugs, must begin within minutes. Anaphylaxis may be caused by extremely small amounts of antigen.

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