Antiphospholipid syndrome (APS) is a type of autoimmune disorder in which antibodies attack the body’s own phospholipids (a type of fat that is an important part of cell membranes). The underlying reason that antibodies are directed at the body's own phospholipids is unknown, and the exact mechanism of action is also unclear. Currently, many researchers believe that antiphospholipid antibodies are first created, for unknown reasons, in response to exposure to an infectious agent, such as a bacteria or virus. The full-blown syndrome develops after a "second hit" that triggers the antibodies. The second hit may include smoking, immobilization, pregnancy, oral contraceptives, hormone replacement therapy, cancer, high blood pressure and high cholesterol.

When this condition occurs by itself, it is referred to as primary APS. If the condition involves multiple organ systems, it is referred to a catastrophic APS, a more serious form of the disease. It is also associated with lupus, certain rheumatic diseases and infections, and drug use. Without treatment, people with APS are at increased risk for:

• Recurrent episodes of deep vein thrombosis (DVT). DVT is the formation of an obstructing blood clot in the deep veins embedded in the muscles, usually in the lower leg and sometimes in the lower abdomen or groin. Though DVT may cause swelling and pain in the leg, it may not cause any symptoms. DVT carries a high risk of either the whole blood clot or a piece of the blood clot breaking off and traveling through the bloodstream. If the traveling material lodges in one of the arteries of the lungs, then a potentially fatal pulmonary embolism could occur.
  
  
• Recurrent miscarriages. Since APS was identified, treatment has significantly improved a pregnant APS patient’s chances of carrying a fully developed, healthy baby to term.
  
  
• The formation of blood clots in an artery. If blood clots form in the cerebral arteries leading to the brain, the patient is at increased risk of stroke – a life-threatening event in which the blockage prevents adequate blood flow to part of the brain. If blood clots form in the coronary arteries leading to the heart, the patient is at increased risk of a heart attack – a life–threatening event that results in permanent heart damage or death. Blood clots may also form in the peripheral arteries in the arms, legs and other noncardiac areas of the body (peripheral arterial disease).
  
  
  
• Thickening and other deformities of heart valves.
  
  
• Pulmonary hypertension, usually caused by pulmonary embolism caused by the formation of blood clots in the lungs.
  
  
• Nervous system disorders. Individuals with APLS have an increased risk for epilepsy and Guillain-Barré syndrome.
  
  
• Skin disorders. Leg ulcerations and “splinter hemorrhages” (bleeding under the fingernails or toenails) can be associated with APLS.
  
  
• Kidney disorders.
  
  
• Blood disorders, including low blood platelets (thrombocytopenia) or low red blood cell count (anemia).