The protein fibrin is the major component of blood clots.
Fibrin is the protein that holds the clot together.

Fibrin is produced when fibrinogen is cleaved by the enzyme thrombin.
Thrombin also activates several other enzymes of the clotting cascade, a role that will be explored further later in the curriculum.

Thrombin is a serine protease.
Serine proteases are a large class of enzymes that share a common mechanism of action that involves a serine at the catalytic center of the enzyme.
The serine proteases all function to cleave peptide bonds, but differ in substrate specificity.
Each serine protease targets and cleaves a different peptide sequence.

Serine proteases include all of the enzymes of the clotting cascade and several of the digestive tract enzymes including trypsin, chymotrypsin, and pepsin.