Clotting factors are substances (enzymes, coenzymes, reagents) necessary for the clotting stages to happen. Besides those triggering factors and reagents already described (tissue and platelet factors, thromplastinogen, prothrombin, fibrinogen, calcium ions), other substances participate in the blood clotting process as clotting factors, like factor VIII, whose deficiency causes hemophilia A, or the factor IX, whose deficiency causes hemophilia B.   _{Your Comment}