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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, 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. 156-1). 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 156-1

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; LT, leukotriene; PAF, platelet-activating factor; PGD2, prostaglandin D2.

URTICARIA AND ANGIOEDEMA

DEFINITION

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

CLASSIFICATION AND ETIOLOGY

The classification of urticaria-angioedema focuses on mechanisms that elicit clinical disease and can be useful for differential diagnosis, but most cases of chronic urticaria are idiopathic (Table 156-1).

TABLE 156-1CLASSIFICATION OF URTICARIA AND/OR ANGIOEDEMA

Urticaria-angioedema can occur secondary to inhalation, physical contact, or more commonly ingestion (fruits, shellfish, fish, milk products, chocolate, legumes including peanuts, drugs) that may elicit urticaria alone or the anaphylactic syndrome (Chap. 26).

PATHOPHYSIOLOGY

Characterized by edema in the superficial dermis in urticaria and subcutaneous tissue and deep dermis in angioedema. Up to 40% of pts with chronic urticaria have an autoimmune cause ...

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