Immune thrombocytopenia is a disorder characterized by a blood abnormality called thrombocytopenia, which is a shortage of blood cells called platelets that are needed for normal blood clotting. Affected individuals can develop red or purple spots on the skin caused by bleeding just under the skin's surface. Immune thrombocytopenia usually happens when your immune system mistakenly attacks and destroys platelets, which are cell fragments that help blood clot. In adults, this may be triggered by infection with HIV , hepatitis or H. pylori — the type of bacteria that causes stomach ulcers.To diagnose immune thrombocytopenia, your doctor will try to exclude other possible causes of bleeding and a low platelet count, such as an underlying illness or medications you or your child may be taking. Blood tests can check the levels of platelets.