Anaphylaxis was first described in 1902 in a study of immunizing dogs with jellyfish toxin. Instead of generating protection, some animals experienced rapid onset of fatal or near-fatal symptoms. Anaphylaxis is derived from the Greek words a (against) and phylaxis (immunity). In the US, the estimated lifetime prevalence of anaphylactic shock from all causes is 1.6%. Fatal anaphylaxis is rare; however, in developed countries, the prevalence of anaphylactic shock is increasing. Symptoms develop within seconds to minutes of exposure, but rarely, hours may pass before symptom onset. Anaphylaxis can spontaneously resolve due to endogenous mediators like angiotensin II and epinephrine. The course of anaphylaxis is unpredictable, and treatment should not be withheld once symptoms begin. Death from anaphylaxis results from airway obstruction or cardiac arrest. The median time from symptom initiation to cardiac arrest was 5 min in iatrogenic anaphylaxis, 15 min in stinging insect-induced anaphylaxis, and 30 min in food allergy-induced anaphylaxis. Biphasic anaphylaxis (symptoms return after treatment without re-exposure) has been reported at a rate of 15-21% of cases. Biphasic reactions typically occur within the first 12 hours; however, recurrences 72 hours later have been reported. Protracted anaphylaxis occurs when symptoms last for days or weeks.