Abstract:Hereditary angioedema (HAE) causes recurrent episodes of angioedema that may be very severe and are frequently associated with significant morbidity and even mortality. Understanding the pathophysiology of this disease is crucial for proper diagnosis and management of these patients. HAE is caused by mutations in the SERPING1 gene that result in decreased plasma levels of functional C1 inhibitor. A large number of different mutations have been described that result in HAE. About 15% of patients have a mutation at or near the active site of the reactive mobile loop, resulting in a protein that lacks functional activity (type II HAE). Type I HAE is caused by a diverse range of mutations, some of which cause the nascent protein to misfold and thus to be unable to enter the secretory pathway. The primary mediator of swelling in HAE is bradykinin, a product of the plasma contact system. Bradykinin induces increased vascular permeability by activating the bradykinin B2 receptor, which results in phosphorylation of vascular endothelial cadherin. The regulation of both the bradykinin B2 receptor and peptidases that degrade bradykinin may influence HAE disease severity. HAE results from mutations in the SERPING1 gene that lead to a loss of functional C1 inhibitor. Attacks of angioedema result from generation of bradykinin, which acts on bradykinin B2 receptors to enhance vascular permeability.Key Words: HAE, C1 inhibitor, contact system, bradykinin, plasma kallikrein, Hageman factor (WAO Journal 2010; 3:S25-S28) I n 1888, William Osler described a familial form of angioedema associated with significant morbidity and mortality. 1 He called this disease hereditary angio-neurotic edema (HANE), a name that has subsequently been shortened to hereditary angioedema (HAE). The prevalence of HAE is not known for certain, but has been estimated to range from 1:30,000 to 1:80,000 in the general population without any known sex, ethnic, or racial differences. 2 HAE is clinically characterized by recurrent episodes of angioedema that typically involve the extremities, gastrointestinal tract, face, oropharynx, larynx, or external genitalia. Patients with HAE experience discrete episodes of nonpruritic nonpitting angioedema at these sites. 2,3 HAE attacks are classically distinguished from allergic or idiopathic angioedema by their longer duration (typically 72-96 hours), absence of accompanying urticaria and failure to respond to antihistamines or corticosteroid therapy.Most often, patients with HAE become symptomatic during childhood. Fifty percent of HAE patients first experience a swelling episode before age 10, with some patients manifesting angioedema as early as 1 year of age. 4,5 Most patients then experience a worsening of symptoms around puberty. 3 Occasionally, patients with HAE do not begin to show evidence of angioedema until their late teens or early adulthood. Rare patients with HAE have been reported who never experience angioedema, but are identified through screening family members of a symptomatic patient. 5...