Thrombophilia is caused by abnormalities in blood consistency, which is determined by the levels of coagulation factors and other circulating blood proteins that participate in the "coagulation cascade".Normal coagulation is initiated by the release of tissue factor from damaged tissue. Tissue factor binds to circulating factor VIIa. The combination activates factor X to factor Xa and factor IX to factor IXa. Factor Xa (in the presence of factor V) activates prothrombin into thrombin. Thrombin is a central enzyme in the coagulation process: it generates fibrin from fibrinogen, and activates a number of other enzymes and cofactors (factor XIII, factor XI, factor V and factor VIII, TAFI) that enhance the fibrin clot. The process is inhibited by TFPI (which inactivates the first step catalyzed by factor VIIa/tissue factor), antithrombin (which inactivates thrombin, factor IXa, Xa and XIa), protein C (which inhibits factors Va and VIIIa in the presence of protein S), and protein Z (which inhibits factor Xa).In thrombophilia, the balance between "procoagulant" and "anticoagulant" activity is disturbed. The severity of the imbalance determines the likelihood that someone develops thrombosis. Even small perturbances of proteins, such as the reduction of antithrombin  to only 70–80% of the normal level, can increase the thrombosis risk; this is in contrast with hemophilia, which only arises if levels of coagulation factors are markedly decreased.In addition to its effects on thrombosis, hypercoagulable states may accelerate the development of atherosclerosis, the arterial disease that underlies myocardial infarction and other forms of cardiovascular disease.
Thrombophilia (sometimes called hypercoagulability or a prothrombotic state) is an abnormality of blood coagulation that increases the risk of thrombosis (blood clots in blood vessels). Such abnormalities can be identified in 50% of people who have an episode of thrombosis (such as deep vein thrombosis in the leg) that was not provoked by other causes. A significant proportion of the population has a detectable thrombophilic abnormality, but most of these develop thrombosis only in the presence of an additional risk factor.There is no specific treatment for most thrombophilias, but recurrent episodes of thrombosis may be an indication for long-term preventive anticoagulation. The first major form of thrombophilia to be identified by medical science, antithrombin deficiency, was identified in 1965, while the most common abnormalities (including factor V Leiden) were described in the 1990s. Signs and symptoms
The most common conditions associated with thrombophilia are deep vein thrombosis (DVT) and pulmonary embolism (PE), which are referred to collectively as venous thromboembolism (VTE). DVT usually occurs in the legs, and is characterized by pain, swelling and redness of the limb. It may lead to long-term swelling and heaviness due to damage to valves in the veins. The clot may also break off and migrate (embolize) to arteries in the lungs.