The word 'anaphylaxis' is derived from Greek and means 'against protection'. The term was first used by Portier and Richet to describe a phenomenon observed when Portuguese man of war venom induced an acute allergic state rather than immunization or prophylaxis in dogs. The first recorded case of anaphylaxis, death due to the sting of a wasp, is described in the tomb of King Menes in Egypt.

Anaphylaxis is a normal inflammatory response designed to render foreign substances inactive and to amplify the response to recruit other immunosurveillance cells. It becomes a life-threatening reaction when patients recognize an antigen as foreign and the host response is exaggerated to produce acute injury. The reason why patients become sensitized and why they respond in such a pathophysiological manner is still unknown.

The study of human anaphylaxis is exceedingly complex. The major reasons for this difficulty are that it occurs infrequently, prior sensitization to a specific antigen is not often recognized, the clinical manifestations may be produced by a number of both immunological and non-immunological mechanisms, and severe anaphylaxis has an explosive, unpredictable, and catastrophic nature with an associated rapid resolution that usually accompanies therapy.

In immunological terms 'anaphylaxis' signifies a Gell and Coombs type I hypersensitivity reaction that is mediated by IgE or IgG antibodies. In a clinical sense 'anaphylaxis' is used to describe reactions which are severe and resemble the classical features of severe type I hypersensitivity irrespective of mechanism. It is generally accepted that 'anaphylaxis' can be due to physical factors such as physical activity (i.e. exercise) that can cause mobilization of inflammatory cells (i.e. mast cells and basophils).

Synonyms for 'anaphylactic' in its immunological sense include 'reaginic', 'allergic', and 'hypersensitive', and non-immunological mechanisms producing the clinical features characteristically associated with anaphylaxis are called 'allergoid', 'anaphylactoid', 'direct histamine release', or 'pseudoallergic'. Here, the expression clinical anaphylaxis will be used to describe the clinical syndrome irrespective of mechanism. Anaphylaxis will be used to refer to type I hypersensitivity, and anaphylactoid will be used to refer to reactions in which there is no possibility of an immune basis.

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