This is an open-label long-term multicenter phase 3 trial to evaluate the efficacy and safety of ARGX-113 in adult patients with primary ITP.

Inclusion criteria:

1. Ability to understand the requirements 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).
2. Patients enrolled in the ARGX-113-1801 trial who completed the 24-weeks trial period.
3. Women of childbearing potential must have a negative urine pregnancy test at baseline before trial medication (infusion) can be administered.
4. Women of childbearing potential should use a highly effective or acceptable method of contraception during the trial and for 90 days after the last administration of the IMP. They must be on a stable regimen, for at least 1 month (as listed in the protocol)

6. 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).

7. Patient has completed a 52-week treatment period.

Exclusion criteria:

1. Introduction or continuation of non-permitted medications during the ARGX-113-1801 trial (such as anti-CD20 therapy, romiplostim, monoclonal antibodies, Fc fusion proteins or live/live-attenuated vaccines).
2. Pregnant or lactating women, and those intending to become pregnant during the trial or within 90 days after the last dosing.
3. Patients with known medical history of hypersensitivity to any of the ingredients of efgartigimod.
4. Use of any other investigational drug or participation in any other investigational trial.