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Remember, the precursor protein fibrinogen was circulating in the blood all along. It needs to be cut into pieces to form fibrin.

An enzyme, thrombin, can do the cutting. But because you don't want clots to form any old time, the thrombin in your blood is also an inactive form which must be activated before it can make a clot form.

If thrombin activated at the wrong time, a clot could form and lead to a heart attack or stroke, killing the person. This is probably why thrombin activation is a very complicated process, requiring a lot of proteins to interact. The more complex it is, the less likely that a clot will form by accident! The proteins that activate thrombin are called clotting factors or procoagulation factors (like 'Factor VIII'). Most of them are made in the liver, and require Vitamin K for their synthesis.

A series of reactions like this is called a CASCADE.

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