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Factor VIII (antihaemophilic factor) One of the blood clotting factors. Factor VIII is a soluble protein that stimulates the activation of Factor X by Factor IXa, which in turn converts prothrombin to thrombin, thus causing the fibrin matrix of a blood clot to form. Haemophilia is due to a deficiency or defect of Factor VIII and is treated by administration of blood plasma or plasma concentrate containing the factor. Factor VIII can now be obtained from genetically engineered cell cultures. Such preparations avoid the risk of contamination with viruses, notably HIV (the AIDS virus).