Thrombocytopenia is a condition in which you have a low blood platelet count. Platelets (thrombocytes) are colorless blood cells that help blood clot. Platelets stop bleeding by clumping and forming plugs in blood vessel injuries. When you don't have enough platelets in your blood, your body can't form clots. A low platelet count may also be called thrombocytopenia. This condition can range from mild to severe, depending on its underlying cause. For some, the symptoms can include severe bleeding and are possibly fatal if they're not treated.