Anaphylaxis may occur within seconds or be delayed over an hour after an exposure; more rapid reactions are associated with higher mortality. Common exposures are foods, medications, insect stings, and allergen immunotherapy injections. Many cases are idiopathic. Criteria for anaphylaxis describe an acute progression of organ system involvement that may lead to cardiovascular collapse. Organ system involvement can include dermatologic (pruritus, flushing, urticaria, erythema multiforme, angioedema), respiratory tract (dyspnea, wheezing, cough, stridor, rhinorrhea), cardiovascular (dysrhythmias, collapse, arrest), gastrointestinal (cramping, vomiting, diarrhea), genitourinary (urgency, cramping), and eye (pruritus, tearing, redness). A biphasic mediator release can occur in up to 20% of cases causing recurrence of symptoms 4 to 8 hours after the initial exposure. Patients on β-blockers are susceptible to an exaggerated allergic response and may be refractory to first line treatment.