The exact cause of ITP isn't known. That's why it's referred to as idiopathic, which means "of unknown cause." It is known, however, that in people with idiopathic thrombocytopenic purpura, the immune system malfunctions and begins attacking platelets as if they were foreign substances.

Antibodies produced by your immune system attach themselves to the platelets, marking the platelets for destruction. The spleen, which helps your body fight infection, recognizes the antibodies and removes the platelets from your system. The result of this case of mistaken identity is a lower number of circulating platelets than is normal.

A normal platelet count is generally higher than 150,000 platelets per microliter of circulating blood. People with ITP often have platelet counts below 20,000. As the number of platelets decreases, your risk of bleeding increases. The greatest risk is when your platelet count falls very low — below 10,000 platelets per microliter. At this point, internal bleeding may occur despite a lack of any injury.

In most children with ITP, the disorder follows a viral illness, such as the mumps or the flu. It may be that an infection sets off the immune system, triggering it to malfunction.

Dec. 10, 2014