What Are the Causes of ITP

The underlying cause of ITP is unknown. We do not know what triggers the immune system to start attacking platelets in the first place, but we have learned a lot about how platelets are being attacked. To understand what's happening to platelets in an adult patient with ITP, you have to understand a little about what platelets are and how they are made.¹

Platelets are small, disc-shaped cells that are made in the bone marrow by cells called megakaryocytes (meg-ah-carry-oh-sights). Platelets enter the bloodstream and circulate. When blood vessels are damaged, platelets help to form clots in order to stop bleeding.²

Typically, each platelet survives for about 9 or 10 days before being removed from the bloodstream. In an adult patient with ITP, antibodies attack platelets in the bloodstream, causing them to be removed and destroyed much sooner, perhaps as soon as a few hours after entering the circulation. Antibodies may also attack the megakaryocytes themselves. This may prevent these cells from making as many platelets as they typically would have.³

The liver produces a protein called thrombopoietin (TPO) that controls how many platelets are made by megakaryocytes. When platelet levels are normal, just enough TPO gets to the bone marrow to keep platelet counts level. If there are too few platelets in the bloodstream, more TPO is able to get to the bone marrow, and more platelets are produced.

With ITP, however, there usually isn't enough TPO getting to the bone marrow. This lower level of TPO means that there are not enough platelets being produced to make up for what has been destroyed.⁴