The main purpose of this study is to look at the effect (efficacy) and safety of efgartigimod IV in participants with primary immune thrombocytopenia (ITP). After an up to 2 weeks screening period, eligible participants will be randomized in a 2:1 ratio to receive either efgartigimod IV or placebo IV, respectively during the double-blinded treatment period (DBTP). At the end of the treatment period (up to 24 weeks), all participants will receive efgartigimod IV during the first 52-week open-label treatment period (OLTP1). At the end of the first OLTP1, participants may begin a second 52-week OLTP2. After the OLTP2, the participants will enter a follow-up period (approximately 8 weeks) while off study drug. The participants will be in the study for up to 138 weeks.

More information can be found here: https://clinicaltrials.argenx.com/advancenext

Inclusion Criteria:

• Is at least 18 years of age and the local legal age of consent for clinical studies when signing the informed consent form (ICF).
• Has documented baseline mean platelet count of <30 x 10^9/L before randomization
• Has a documented duration of primary immune thrombocytopenia (ITP) of more than 12 months on the date of informed consent form (ICF) signature.
• Has documented prior ITP treatment with at least 1 of the following treatments: corticosteroids, intravenous immunoglobulin (IVIg), anti-D immunoglobulin, thrombopoietin receptor agonist (TPO-RAs), or rituximab.
• Has documented insufficient response to a prior ITP treatment (the specific criteria can be found in the protocol).
• Has documented prior response defined as 1 platelet count of ≥50 × 109/L to at least 1 of the following ITP treatments in the 3 years before the date of ICF signature: prednisone, dexamethasone, other or nonspecified corticosteroids, IVIg, or anti-D immunoglobulin

Exclusion Criteria:

• Other than the indication under study, known autoimmune disease or any medical condition that would interfere with an accurate assessment of clinical symptoms of ITP, confound the results of the study or put the participant at undue risk.
• Secondary ITP
• Nonimmune thrombocytopenia
• Autoimmune hemolytic anemia
• ITP-associated critical or severe bleeding The complete list of criteria can be found in the protocol.