Antiphospholipid syndrome is a disorder characterized by an increased tendency to form abnormal blood clots (thromboses) that can block blood vessels. This clotting tendency is known as thrombophilia. In antiphospholipid syndrome, the thromboses can develop in nearly any blood vessel in the body. If a blood clot forms in the vessels in the brain, blood flow is impaired and can lead to stroke. Antiphospholipid syndrome is an autoimmune disorder. Autoimmune disorders occur when the immune system attacks the body's own tissues and organs.