Antiphospholipid Syndrome (APS) is an autoimmune disorder that can cause blood clots and pregnancy complications. It occurs when the immune system mistakenly produces antibodies that attack phospholipids, a type of fat found in the membranes of cells and in the blood. Immune system normally makes proteins called antibodies to protect you from unhealthy invaders like viruses. But when you have antiphospholipid syndrome, it makes antibodies that attack fat molecules throughout your body. That leads to cell damage, which makes your blood more likely to form clots in arteries and veins. The clots can form nearly anywhere in your body, from your brain to your lungs to your legs.

The exact cause of APS is not fully understood, but it is believed to involve a combination of genetic predisposition and environmental triggers and infections. Many autoimmune diseases also may be associated with APS.