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Overview of Allergic and Atopic Disorders - Immunology; Allergic Disorders - Merck Manual Professional Edition

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Allergic (including atopic) and other hypersensitivity disorders are inappropriate or exaggerated immune reactions to foreign antigens. Inappropriate immune reactions include those that are misdirected against intrinsic body components (self), leading to autoimmune disorders. This topic focuses on type I hypersensitivity reactions. Hypersensitivity reactions are divided into 4 types (I through IV) by the Gell and Coombs classification. Hypersensitivity disorders often involve more than 1 type. Type I reactions (immediate hypersensitivity) are IgE-mediated. Antigen binds to IgE that is bound to tissue mast cells and blood basophils, triggering release of preformed mediators (eg, histamine, proteases, chemotactic factors) and synthesis of other mediators (eg, prostaglandins, leukotrienes, platelet-activating factor, cytokines). These mediators cause vasodilation, increased capillary permeability, mucus hypersecretion, smooth muscle spasm, and tissue infiltration with eosinophils, type 2

Overview of Allergic and Atopic Disorders - Allergy and Immunology - Merck Manual Professional Edition honeypot link Overview of Allergic and Atopic Disorders Full Review: Aug 2024 By James Fernandez , MD, PhD , Cleveland Clinic Lerner College of Medicine at Case Western Reserve University | Peer reviewed by Brian F. Mandell , MD, PhD , Cleveland Clinic Lerner College of Medicine at Case Western Reserve University Last updated: Apr 2026 v994881 View Patient Education Classification of Hypersensitivity Reactions | Etiology | Pathophysiology | Symptoms and Signs | Diagnosis | Treatment | Prevent

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