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Harvard Medical School

Immune Thrombocytopenic Purpura (ITP)

What Is It?

Immune thrombocytopenic purpura (ITP) can be understood by looking at the three terms that make up its name:

• Immune indicates that the illness is caused by the immune system, which makes cells and antibodies that attack the person's own platelets — the parts of the blood that help the blood to clot.

• Thrombocytopenic means that the illness is related to low levels of thrombocytes, another name for platelets.

• Purpura means that the illness produces a red or purple rash that is caused by bleeding under the skin.

In short, ITP is an illness in which unusually low levels of platelets lead to purpura and other forms of abnormal bleeding.

In people with ITP, the immune system produces abnormal proteins called antiplatelet antibodies. These misdirected proteins attach themselves to the surface of blood platelets as if the platelets were "foreign" or invading bacteria or viruses. As the affected platelets circulate in the bloodstream, they are recognized as abnormal by the spleen and removed from the blood. As more and more platelets are removed by the spleen, the level of platelets in the blood drops past the lower limit of normal (about 130,000 per cubic millimeter of blood) and the patient is diagnosed with thrombocytopenia. When platelet levels fall into the 30,000 to 50,000 range, a person may begin to have abnormal bleeding after a minor skin injury, such as a small cut, bruise, medical injection, blood test or tooth extraction. If platelet levels fall below 10,000, the person has an increased risk of bleeding even when no injury has occurred. This type of bleeding is especially dangerous if it happens inside the skull, where it is known as an intracranial hemorrhage.

There are two types of ITP: acute and chronic.

• Acute ITP — This form of ITP lasts for less than six months and typically affects children, most commonly those between the ages of 2 and 6. It usually appears shortly after a viral infection. Most children with acute ITP recover without treatment, and their platelet counts eventually rise to normal levels. However, 7% to 28% of people with acute ITP go on to develop chronic ITP.

• Chronic ITP — This form of ITP lasts for more than six months, usually strikes adults between the ages of 20 and 40, and requires medical treatment to restore normal platelet levels. Chronic ITP is three times more common in women than men. Typically, a person has weeks or months of mild to moderate abnormal bleeding off and on before seeing a doctor.

In most cases, the cause of ITP is unknown. In a few cases, ITP can be a reaction to a specific drug or food ingredient, such as quinine in tonic water. In the United States, ITP is a relatively rare illness that affects about four out of every 100,000 people each year.