Anaphylaxis is a severe, sometimes life-threatening, allergic reaction that occurs within minutes to several hours of exposure an allergy-causing substance (allergen). Anaphylaxis also is called anaphylactic shock.
In an allergic reaction, the body's immune system responds to the presence of an allergen by releasing histamine and other body chemicals. These chemicals cause the symptoms of allergies, which are usually mild but annoying, such as the runny nose of hay fever (allergic rhinitis) or the itchy rash of poison ivy. However, in some cases, the symptoms can be much worse and involve the entire body.
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