Untreated person with HAE during an attack
Most cases of HAE are caused by an insufficient amount of or defective C1 esterase inhibitor, a protein in the blood that helps to prevent swelling experienced during HAE attacks. The lack of C1-INH may contribute to a cascade of events in the body. This cascade causes another protein, called plasma kallikrein, to trigger the release of a third protein, called bradykinin. Having too much bradykinin causes the blood vessels to release fluid, resulting in localized swelling.