The purpose of this study is to evaluate efficacy, safety and tolerability of blisibimod when administered on top of standard-of-care to subjects with Immune Thrombocytopenic Purpura (ITP).

Inclusion Criteria:

1. 18 to 75 years of age(male or female).
2. Diagnosis of ITP according to the guidelines of the American Society of Hematology (ASH) and British Committee for Standards in Hematology.
3. Platelet counts at Screening of 30 billion/L or less for subjects not on ITP medication, or 50 billion/L or less for subjects receiving stable background ITP medication.

Exclusion Criteria:

1. Subjects who have had a splenectomy for any reason.
2. Currently receiving high-dose ITP medications, eltrombopag, romiplostim, rituximab, or investigational therapeutic agents.
3. Nursing or pregnant.
4. Active infection requiring hospitalization or treatment with parenteral antibiotics within the past 60 days.
5. Any known history of bone marrow stem cell disorder.
6. Active hepatitis B, active hepatitis C or a documented history of HIV, hepatitis B, or hepatitis C.
7. Liver disease.
8. Malignancy within the past 5 years.
9. History of active tuberculosis (TB) or history of TB infection.
10. Subject has not yet completed at least 3 months or 5 half-lives (whichever is longer) since ending other investigational study.
11. History of congenital immunodeficiency.