Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 18.0-75.0,  Acquired Thrombotic Thrombocytopenic Purpura (aTTP) Participant or legally authorized representative voluntarily signs informed consent. For participants unable to provide consent, a fully recognized medical proxy may be used according to local laws Participant is 18 to 75 years old at the time of screening Participant has been diagnosed with primary or secondary autoimmune acquired thrombotic thrombocytopenic purpura (aTTP) based on the following a) Thrombocytopenia [drop in platelet count greater than or equal to (>=) 50% or platelet count lesser than (<) 100,000/microlitre (μL)] i) No more than 3 participants per arm may be enrolled with a screening platelet count >= 50,000/μL. b) Microangiopathic hemolytic anemia [elevation of lactate dehydrogenase (LDH) greater than (>) 2-fold or by presence or increase of schistocytes in peripheral blood smear] Willingness to fully comply with study procedures and requirements, and intention to initiate plasma exchange (PEX). Participants may be provisionally entered into the trial and undergo randomization pending the results of the activity, anti-ADAMTS-13 antibody, and genetic testing for congenital thrombotic thrombocytopenic purpura (cTTP) If female of childbearing potential, participant presents with a negative pregnancy test and agrees to employ adequate birth control measures for the duration of the study. Sexually active males must use an accepted and effective method of contraception during the treatment and until a minimum of 16 days after the last dose administered  Participant has been diagnosed with congenital TTP Participant has plasma activity > 10% of normal at the central lab; if screening samples are not taken until after the first PEX, activity from the local lab is permitted to determine eligibility Participant has been diagnosed with another cause of thrombotic microangiopathy (TMA) including: DIC, disseminated malignancy, malignant hypertension, hematopoietic stem cell transplantation, shiga toxin related and atypical HUS, drug toxicity (e.g. gemcitabine, mitomycin C, clopidogrel) and pregnancy-related thrombocytopenia syndromes (e.g. HELLP, eclampsia) Participant has been exposed to another IP within 30 days prior to enrollment or is scheduled to participate in another clinical study involving IP or investigational device during the course of the study Participant has received caplacizumab within 1 month prior to study enrollment Participant is human immunodeficiency virus positive (HIV+) with unstable disease or CD4+ count lesser than or equal to (<=) 200 cells/mm3 within 3 months screening Participants with conditions of severe immunodeficiency Participant has had a previous aTTP event in the past 30 days Participant has another underlying progressive fatal disease and/or life expectancy of less than 3 months Participant is identified by the investigator as being unable or unwilling to cooperate with study procedures