Anaphylaxis is a rare, generalized, potentially life-threatening allergic reaction to a particular substance (allergen) to which individuals have previously developed an extreme sensitivity (hypersensitivity). The reaction typically occurs within seconds or minutes or, more rarely, up to a few hours after exposure to such an allergen. Allergens may include insect venom, certain foods, medications, vaccines, chemicals, or other substances. An anaphylactic reaction may be characterized by development of an itchy, reddish rash (hives); a severe drop in blood pressure; swelling and obstruction of the mouth, nose, and throat; abdominal cramps; nausea and vomiting; diarrhea; and severe difficulties breathing. Without immediate, appropriate treatment, the condition may rapidly lead to a state of unconsciousness (coma) and life-threatening complications.