On the one hand, C1-INH deficiency leads to reduced control of the complement [7,8], and on the other hand, it leads to kallikrein–kinin system (KKS) activation, with subsequent kininogen cleavage and accumulation of kinins on endothelial cells, including bradykinin, the most predominant mediator of vascular permeability in angioedema attacks. Here, KLK4 is linked to angioedema.