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Causes
By Mayo Clinic staffA blood clot (thrombus) normally forms to stop the bleeding when an artery or vein is damaged, such as when you experience a cut. Clots form as a result of chemical reactions between specialized blood cells (platelets) and proteins in your blood (clotting factors). Anti-clotting factors control excessive formation of blood clots. One of the clotting proteins is factor V. People with factor V Leiden have a genetic mutation that results in factor V protein responding more slowly to the anti-clotting factors.
In the normal clotting process, anti-clotting proteins combine to help break up factor V to keep it from being reused and forming clots when clotting isn't needed. However, the factor V Leiden mutation keeps the anti-clotting proteins from breaking down factor V, which keeps it in the blood longer and increases the chance of clotting.
If you have factor V Leiden, you either inherited one copy of the defective gene (heterozygous), which slightly increases your risk of developing blood clots, or you inherited two copies, one from each parent (homozygous), which significantly increases your risk.
