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INTRODUCTION

DEFINITION

These diseases result from IgE-dependent release of mediators from sensitized basophils and mast cells upon contact with an offending antigen (allergen). Associated disorders include anaphylaxis (Chap. 28), allergic rhinitis, urticaria, asthma, and eczematous (atopic) dermatitis.

PATHOPHYSIOLOGY

IgE binds to the surface of mast cells and basophils through a high-affinity receptor. Cross-linking of this IgE by antigen causes cellular activation with subsequent release of preformed and newly synthesized mediators (Fig. 159-1). These mediators have been implicated in many pathophysiologic events associated with immediate-type hypersensitivity, such as vasodilation, increased vasopermeability, smooth-muscle contraction, and chemotaxis of neutrophils and other inflammatory cells. The clinical manifestations of each allergic reaction depend largely on the anatomic site(s) and time course of mediator release.

FIGURE 159-1

Bioactive mediators of three categories generated by IgE-dependent activation of murine mast cells can elicit common but sequential target cell effects leading to acute and sustained inflammatory responses. GM-CSF, granulocyte-macrophage colony-stimulating factor; IFN, interferon; IL, interleukin; LT, leukotriene; PAF, platelet-activating factor; PGD2, prostaglandin D2; TNF, tumor necrosis factor.

URTICARIA AND ANGIOEDEMA

DEFINITION

May occur together or separately. Urticaria involves only the superficial dermis and presents as pruritic, circumscribed wheals with raised serpiginous borders and blanched centers. Angioedema involves deeper layers of skin and may include subcutaneous tissue; it is marked by dramatic swelling with more pain than pruritus. Recurrent episodes of urticaria and/or angioedema of <6 weeks duration are considered acute, whereas attacks persisting beyond this period are chronic.

CLASSIFICATION, ETIOLOGY, AND PATHOPHYSIOLOGY

The classification of urticaria-angioedema focuses on mechanisms that elicit clinical disease and can be useful for differential diagnosis (Table 159-1). Acute urticaria is most often the result of exposure to a food, environmental or drug allergen or viral infection. Chronic urticaria is often idiopathic with additional etiologies including physical stimuli. Up to 45% of pts with chronic urticaria have an autoimmune cause including autoantibodies to IgE or to the α chain of FcεRI. Hereditary angioedema (HAE) is a fully penetrant, autosomal dominant disease due to a mutation in the SERPING1 gene leading to a deficiency of C1 inhibitor (C1INH) (type 1—85% of pts) or to a dysfunctional protein (type 2).

Table 159-1Classification of Urticaria and/or Angioedema

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