Anaphylaxis is a systemic immediate hypersensitivity reaction characterized by edema in many tissues and a fall in blood pressure, secondary to vasodilation. These effects usually result from the systemic presence of antigen introduced by injection, an insect sting, or absorption across an epithelial surface such as the skin or gut mucosa. The allergen activates mast cells in many tissues, resulting in the release of mediators that gain access to vascular beds throughout the body. The decrease in vascular tone and leakage of plasma caused by the released mediators can lead to a fall in blood pressure or shock, called anaphylactic shock, which is often fatal. The cardiovascular effects are accompanied by constriction of the upper and lower airways, laryngeal edema, hyper-motility of the gut, outpouring of mucus in the gut and respiratory tract, and urticarial lesions (hives) in the skin. It is not known which mast cell mediators are the most important in anaphylactic shock. The mainstay of treatment is systemic epinephrine, which can be lifesaving by reversing the bronchoconstrictive and vasodilatory effects of mast cell mediators. Epinephrine also improves cardiac output, further aiding survival from threatened circulatory collapse. Antihistamines may also be beneficial in anaphylaxis, suggesting a role for histamine in this reaction. In some animal models, PAF receptor antagonists offer partial protection.
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