This blood test checks for antiphospholipid antibodies. These may be found in people with abnormal blood clots or autoimmune diseases. Your immune system usually creates antibodies in response to an infection or foreign invaders like bacteria. Antiphospholipid antibodies are usually made when your immune system mistakes part of your own body for a harmful substance. In this case, the antibodies seem to be reacting to phospholipids. Phospholipids are a normal part of your blood vessels. People who have abnormal blood clots, repeated miscarriages, or autoimmune diseases, such as systemic lupus erythematosus (SLE) and multiple sclerosis often have antiphospholipid antibodies.