Definition

Anaphylaxis means a severe systemic allergic reaction. No universally accepted definition exists because anaphylaxis comprises a constellation of features, and the argument arises over which features are essential features. A good working definition is that it involves one or both of two severe features: respiratory difficulty (which may be due to laryngeal oedema or asthma) and hypotension (which can present as fainting, collapse, or loss of consciousness). Other features are usually present.

The confusion arises because systemic allergic reactions can be mild, moderate, or severe. For example, generalised urticaria, angio-oedema, and rhinitis would not be described as anaphylaxis, as neither respiratory difficulty nor hypotension—the potentially life threatening features—is present.

Mechanism

An allergic reaction results from the interaction of an allergen with specific IgE antibodies, bound to Fc receptors for IgE on mast cells and basophils. This leads to activation of the mast cell and release of preformed mediators stored in granules (including histamine), as well as of newly formed mediators, which are synthesised rapidly. These mediators are responsible for the clinical features. Rapid systemic release of large quantities of mediators will cause capillary leakage and mucosal oedema, resulting in shock and asphyxia.