Children are eligible if admitted to a pediatric department in Denmark for diagnosis, observation or treatment of acute or chronic ITP. Examination and diagnostic work up is similar throughout the country, but not identical. No specific tests are required for diagnosis. If treatment is indicated rhesus positive children are treated with subcutaneous anti-D. Rhesus negative children are treated according to local guidelines. Specified follow-up on all children is mandatory. For research purposes one blood sample form all children is collected, and from children, who receive medical treatment, several blood samples are collected. Analysis for changes in immunological signaling peptides will be performed with special attention to the mechanism of action of anti-D.