Antiphospholipid syndrome (APS) is an autoimmune disorder in which the immune system attacks a special kind of fat cell called a phospholipid. These cells are an important part of cell membranes. An autoimmune disorder is one in which the immune system mistakenly identifies normal, "self" tissues as "non-self" antigens and mounts an immune response against them in an attempt to destroy them.

APS is characterized by a number of problems, including increased tendency for blood clots and stroke, heart valve disorders, miscarriages, and low platelet counts (thrombocytopenia). The risk of these problems is significantly reduced with treatment, which includes anticoagulants and aspirin. Because it is unclear why the condition develops, preventive strategies are unknown.