Anaphylaxis is a serious allergic reaction that is rapid in onset and may cause death. There are three main classifications of anaphylaxis.

Anaphylactic shock is associated with systemic vasodilation that causes low blood pressure which is by definition 30% lower than the person’s baseline or below standard values.

Biphasic anaphylaxis is the recurrence of symptoms within 1–72 hours with no further exposure to the allergen.