A rare condition in which blood clots form in small blood vessels throughout the body. These clots can block the flow of blood and oxygen to the body’s organs, which leads to a lower-than-normal number of platelets in the blood. Signs and symptoms of aTTP include purple bruises or tiny red or purple spots on the skin, bleeding problems, anemia, fever, chest pain, nervous system problems, and kidney problems. aTTP occurs when the body starts producing antibodies that block an enzyme called ADAMTS13, which is involved in blood clotting. It may be brought on by certain diseases or conditions (such as pregnancy, cancer, lupus, HIV, or other infections), certain medicines (such as chemotherapy or hormone therapy), or certain procedures (such as surgery or a stem cell transplant). aTTP usually occurs in adults. Also called acquired thrombotic thrombocytopenic purpura.