In ITP, antibodies are usually attached to platelets and are eliminated from the body in a number of ways. Researchers have studied each step, from platelet production to platelet elimination, and continue to find subtleties in what was once thought to be a simple and straight-forward process.
Platelet Production
Platelets are released from megakaryocytes, cells in the bone marrow, in an elegant process that includes budding and creating a long chain of pro-platelets. The megakaryocytes can also release groups of platelets. In people with ITP the platelets are often slightly enlarged since they are newly formed.
Watch a video of the platelet production process:
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Thrombopoietin
Thrombopoietin (TPO) is a protein made in the liver that travels through the blood stream and signals the bone marrow to make more platelets. In people with ITP, TPO is not as elevated as you would expect or as much as it is in some other diseases that are accompanied by low platelets.
Platelet Function
Platelets are very active particles in the blood, responsible for assuring the integrity of the blood vessel walls. They plug up small holes and form clots when vessel walls are injured.
Platelets also transport other substances throughout the blood stream. In the body, 2% of the serotonin, a mood elevating neurotransmitter, is stored in platelets. In addition to serotonin, platelets also carry its 'parent' or precursory chemical L-tryptophan. These substances are involved in such processes as sleep/wake cycles, biological rhythms, appetite, mood regulation, etc.
Platelet Antibodies
Antibodies may attach to platelets because the platelets are damaged, because parts of the platelets look like a virus or bacteria that the body is fighting, because of a defect in the immune system, or for other reasons.
In most people with ITP, antibodies attach to their platelets at glycoprotein 2A/3B. However, the antibodies can attach to other parts of their platelets as well. While researchers have tried very hard to create a laboratory test to diagnose ITP by measuring the platelet antibodies, this has not been successful since some people with ITP have no detectable antibodies and some people with a normal platelet count have anti-platelet antibodies according to these diagnostic tests.
Platelet Lifespan
Platelets stay in the blood stream from a few hours to close to the normal eight to ten days, depending on the severity of the disease. The antibody-coated platelets are eliminated by macrophages in the spleen and sometimes, the liver. Platelets can also be removed by other components of the blood stream.
