This trial is designed as a multi-centre, single-dose, exploratory dose-finding, open label trial evaluating the safety and efficacy of Sym001 in 4-9 consecutive cohorts. Subjects will receive a single IV dose of Sym001.

Inclusion Criteria:

• Confirmed presence of thrombocytopenia with platelet count < 30,000/mm3 at the pre-dose visit.
• History of isolated ITP
• RhD-positive serology.
• Previous treatment and response to first line therapy for ITP

Exclusion Criteria:

• Known clinical picture suggestive of other causes of thrombocytopenia, especially systematic lupus erythematosus,antiphospholipid syndrome, Evans syndrome, immunodeficiency states, lymphoproliferative disorders, liver disease,ingestion of drugs such as quinidine/quinine, heparin and sulfonamides and hereditary thrombocytopenia confirmed by relevant laboratory findings.
• Suspected infection with HIV, Hepatitis C, H. pylori.
• Clinical splenomegaly
• History of abnormal bone marrow examination.
• Ongoing haemorrhage corresponding to a grade 3 or 4 on the WHO bleeding scale.
• Underlying haemolytic condition
• History of splenectomy.
• Subject is pregnant, breast feeding or intends to become pregnant.