The fibrin clot is stabilized by the activity of which enzyme?

• A. α2-Antiplasmin
• B. Factor XIIIa
• C. Plasmin
• D. Thrombin

Answer: (B)

Fibrin clot stabilization comes from covalent cross-linking of fibrin strands, which strengthens the mesh and makes it more resistant to mechanical disruption and early breakdown. This cross-linking is performed by Factor XIIIa, a transglutaminase activated from Factor XIII by thrombin in the presence of calcium. Once active, XIIIa forms covalent bonds between fibrin molecules, especially linking certain amino acid residues on adjacent fibrin chains, creating a more durable network.

Plasmin is the enzyme that breaks down fibrin, not stabilizes it, while alpha2-antiplasmin inhibits plasmin to slow degradation. Thrombin initiates clot formation and also activates Factor XIII to XIIIa, but the stabilizing action specifically comes from the cross-linking carried out by XIIIa.