Pathophysiology, diagnosis and treatment of anaphylactic and anaphylactoid reactions.

Anaphylaxis is the product of IgE-dependent activation of mast cells and basophils, and results in the production of large amounts of mast cell- and basophil-derived mediators, such as histamine. Anaphylactoid reactions are similar to anaphylactic reactions, but are the result of a non-IgE-dependent activation of mast cells and basophils. The pathophysiological role of histamine in anaphylaxis and anaphylactoid reactions has been assessed in a large number of studies showing that plasma histamine levels and urinary concentrations of methylhistamine increase during anaphylactic and anaphylactoid reactions, and are significantly correlated with the severity of the reaction. Moreover, histamine injections in experimental animals and healthy volunteers have been found to induce the same clinical symptoms and biological alterations as those associated with anaphylaxis. Some clinical manifestations of anaphylaxis such as bronchospasm are due to the direct action of histamine on the cells and tissues, but others, e.g., cardiovascular alterations, are mediated by nitrogen oxide released by cells activated by histamine. Finally, other mast cell- and basophil-derived mediators, such as platelet-activating factor and leukotrienes, may also play a pathophysiological role in anaphylaxis and anaphylactoid reactions. Interactions between the neuroendocrine and immune systems are also involved in anaphylaxis. It has been shown that physiologic and pathologic alterations in the neuroendocrine system favor the onset of anaphylactic reactions, and that the mediators and cytokines involved in anaphylaxis induce alterations in the production of neurohormones. A large number of substances can induce anaphylaxis, including food, drugs and hymenoptera venom. Emergency treatment for severe anaphylactic and anaphylactoid reactions is based on epinephrine injections. The prognosis for these reactions is poor when epinephrine injection is delayed. The frequency of relapse is significantly reduced in patients in whom the substances to which they are allergic or intolerant are known. Thus, allergy tests should be performed in patients with anaphylaxis, to determine the allergens responsible for anaphylactic reactions. Finally, the prevention of relapse is based on allergen eradication (certain foods and drugs), and on hyposensitization (hymenoptera venom). (C) 2000 Editions scientifiques et medicales Elsevier SAS.