Inclusion Criteria:  Must be capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. Previously participated in antecedent studies ARGX-113-2001 or ARGX-113-1705 and are eligible for roll over.  Contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies and:  Male participants:  - Male participants are not allowed to donate sperm from signing the informed consent (ICF) until the end of study.  Female participants:  Women of Child bearing potential (WOCBP) must have a negative urine pregnancy test at baseline before investigational medicinal product (IMP) can be administered.  Exclusion Criteria:  The participant was discontinued early from studies ARGX-113-2001 or ARGX-113-1705, unless the reason for discontinuation from study ARGX-113-1705 was to roll over into study ARGX-113-2002.  a. Participants who, in the investigator's judgment, are not benefiting from efgartigimod IV in study ARGX-113-1705 Part B are not eligible for roll over into ARGX-113-2002.  Are pregnant or lactating, or intend to become pregnant during the study or within 90 days after the last dose of investigational medicinal product (IMP)  Has any of the following medical conditions:  Clinically significant uncontrolled chronic bacterial, viral, or fungal infection at screening Any other known autoimmune disease that, in the opinion of the investigator, would interfere with accurate assessment of clinical symptoms of myasthenia gravis or put the participant at undue risk  History of malignancy unless deemed cured by adequate treatment with no evidence of reoccurrence for ≥3 years before the first administration of investigational medicinal product (IMP).  Participants with the following cancers can be included at any time:  adequately treated basal cell or squamous cell skin cancer carcinoma in situ of the cervix carcinoma in situ of the breast incidental histological findings of prostate cancer (TNM classification of malignant tumors stage T1a or T1b) Clinical evidence of other significant serious diseases, or the participant has had a recent major surgery, or who have any other condition that, in the opinion of the investigator, could confound the results of the study or put the participant at undue risk Received a live-attenuated vaccine within 28 days prior to study entry or plan to receive a live-attenuated vaccine during the study A known hypersensitivity reaction to efgartigimod, rHuPH20, or any of its excipients