KLK4 and hereditary angioedema: Within the kallikrein-kinin cascade, C1-INH is a pivotal inhibitor of factor XIIa and plasma kallikrein, which are key plasma proteases involved in the production of bradykinin, a potent vasodilator.9, 10 The absence of sufficient C1-INH function leads to uncontrolled activation of the kallikrein-kinin pathway and overproduction of bradykinin, the underlying cause of edema and swelling in patients with HAE type 1 or 2 (HAE-1/2).9, 11, 12