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Allergy is an immunologically mediated hypersensitivity reaction to a foreign antigen manifested by tissue inflammation and organ dysfunction. The clinical expression of allergic disease depends on prior immunologic responsiveness, antigen exposure, and genetically influenced host factors such as atopy. Atopic patients have a genetically aggregated predisposition to a limited number of disorders: allergic rhinitis (Chapter 8-11), allergic asthma (Chapter 9-05), atopic dermatitis (Chapter 6-09), and IgE-mediated food allergies. Although these disorders tend to aggregate in families designated as “atopic,” each of these disorders can occur in individuals without personal or familial history of an atopic diathesis. In addition, many mast cell and IgE-dependent disorders (eg, many drug and chemical sensitivities, eosinophilic disorders, mast cell stability syndromes, chronic urticaria) occur equally frequently in atopic and nonatopic persons.

The timing of the onset of clinical allergic syndromes after exposure to a suspected allergen serves as a useful clinical marker on which to base diagnosis and treatment. Reactions will usually be either immediate (generally occurring within 60 minutes after initial exposure), or delayed, appearing after many hours to days or weeks of antigen exposure.

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