Exposure to contact substances, such as collagen, can activate the intrinsic pathway. The exposed collagen is the location where a complex forms between:
- High molecular weight kininogen (HMWK)
- Prekallikrein (also known as Fletcher Factor), which activates to kallikrein
- Factor XII (Hageman Factor)
Together, this three-biochemical-complex, adhered to the collagen binding site, catalyzes the conversion of factor XII to its activated form, XIIa, thereby triggering the intrinsic pathway.
The intrinsic pathway is circled in red in the image below.