part of part of blood coagulation, intrinsic pathway https://github.com/geneontology/go-ontology/issues/30501 See also the biological process term 'blood coagulation, extrinsic pathway ; GO:0007598' and 'blood coagulation, common pathway ; GO:0072377'. A protein activation cascade that initiates blood coagulation, triggered when Factor XII, prekallikrein (PK) and high-molecular-weight kininogen (HK) assemble on a suitable surface or polymer. This results in the reciprocal activation of factor XII to Factor XIIa by kallikrein, and PK to kallikrein by Factor XIIa. The resulting generation of Factor XIIa activates Factor XI to Factor XIa, which then converts Factor IX to Factor IXa. Factor IXa converts Factor X to Xa. Factor Xa then initiates the common pathway. GO:0007597 initiation of blood coagulation cascade, contact pathway Reactome:R-HSA-140837 initiation of blood coagulation cascade, contact activation system protein activation cascade blood coagulation, fibrin clot formation