Activation of the contact system. Factor XII (FXII) can be activated by negatively charged surfaces such as polyphosphates (polyP) derived from activated platelets or neutrophil extracellular traps (NETs). PK can be activated to kallikrein (Kal) by prolylcarboxylase (PRCP) on endothelial cells. Factor XIIa will also activate PK and thereby allows reciprocal activation, resulting in the generation of additional factor XIIa. Factor XIIa can initiate coagulation via the activation of factor XI. Both PK and factor XI are in complex with their cofactor, HK.