Inclusion Criteria:  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). 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, his/her 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. Women of childbearing potential: • As defined in Woman of Childbearing Potential in the protocol, women of childbearing potential must have a negative urine pregnancy test at baseline before trial medication can be administered • Must be on a stable regimen for at least 1 month of a highly effective or acceptable method of contraception during the trial and for 90 days after the last administration of efgartigimod PH20 SC Non-sterilized male participants who are sexually active with a female partner of childbearing potential must use an acceptable method of contraception, ie, a condom from signing the informed consent form (ICF) until the end of the trial. Male participants are not allowed to donate sperm from signing the ICF until the end of the trial. 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). 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). Participant has completed a 52-week treatment period.  Exclusion Criteria:  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) Use of any other investigational drug or participation in any other investigational trial Known hypersensitivity reaction to efgartigimod PH20 SC or any of its excipients 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