A Phase 3 Study to Evaluate the Safety and Efficacy of Efgartigimod PH20 Subcutaneous in Adult Patients With Primary Immune Thrombocytopenia

Inclusion Criteria: 1. Ability to understand the requirements of the trial and provide written informed consent (including consent for the use and disclosure of research-related health information), willing and able to comply with the trial protocol procedures (including attending the required trial visits). 2. Participants enrolled in the ARGX-113-2004 trial who completed the 24-week trial period. Note: If a participant has had an SAE during the ARGX-113-2004 trial, their eligibility should be evaluated by the investigator and the sponsor's trial physician. The decision of enrolling the participant will be evaluated case by case. 3a. Agree to use contraceptives consistent with local regulations and the following: • Female participants of childbearing potential must have a negative urine pregnancy test at baseline before receiving IMP. In addition to the above criteria, for participants who want to continue receiving efgartigimod during an additional 52-week treatment period (only applicable in case efgartigimod is not yet commercially available for patients with primary ITP or available through another patient program for patients with primary ITP), the following criteria apply: 4\. Ability to understand the requirements of the additional 52-week treatment period of the trial, to provide written informed consent (including consent for the use and disclosure of research-related health information), and to comply with the trial protocol procedures (including required trial visits). 5\. Participant has completed a 52-week treatment period. Exclusion criteria: 1. Introduction or continuation of nonpermitted medications during the ARGX-113-2004 trial (such as anti-CD20 therapy, romiplostim, monoclonal antibodies, Fc fusion proteins, or live/live-attenuated vaccines) 2. Use of any other investigational drug or participation in any other investigational trial 3. Known hypersensitivity reaction to efgartigimod PH20 SC or any of its excipients 4. Pregnant or lactating females and those who intend to become pregnant during the trial or within 90 days after last dose of efgartigimod PH20 SC