The kallikrein–kinin system (KKS, contact system, or intrinsic pathway) is composed of three zymogens (PK, factor XI, factor XII) and one cofactor (HK, BK precursor), which are involved in the coagulation pathway and kallikrein–kinin forming pathway, in response to vascular injury, abnormality, and infection. Here, KLK4 is linked to infection.