Anaphylaxis is an acute, multisystem, and severe allergic reaction, known also as type I hypersensitivity reaction. The term comes from the Greek words ana (again) and phylaxis (protection). Anaphylaxis occurs when a person is exposed to a trigger substance, called an allergen, to which they are sensitized. Re-exposure to the trigger substance intensifies what is normally a protective immune response and it become detrimental.

Gell and Coombs classified hypersensitivity reactions into four types:

Type I hypersensitivity reactions are extremely important to recognize and treat because their reactions are immediate; abrupt in onset, usually occurring within 15 to 30 minutes from the time of exposure; and potentially life threatening. This “immediate” reaction assists with the identification of the triggering antigen. Anaphylaxis is related to the action of IgE and other anaphylatoxins, which act to release histamine and other mediators from mast cells during degranulation. Histamine has a number of effects, including the vasodilation of arterioles and constriction of bronchioles in the lungs.