Mechanism of angioedema in first complement component inhibitor deficiency

Since shortly after the discovery that hereditary angioedema resulted from deficiency of first complement component (Cl) inhibitor, the characterization of the mediator of angioedema has been a major goal. However, because C1 inhibitor regulates activation of both the contact and complement systems, identification of the mediator was not immediately accomplished. For a number of years, some studies appeared to indicate involvement of one system, whereas other studies suggested involvement of the other. However, the vast majority of the evidence accumulated over the past several years indicates quite clearly that the major mediator is bradykinin. Therefore, unregulated contact system activation is the defect that leads directly to the development of angioedema.