An Open-label, Uncontrolled Study to Evaluate the Pharmacokinetics, Pharmacodynamics, Safety, and Activity of Efgartigimod PH20 SC in Participants From 2 to Less Than 18 Years of Age With Generalized Myasthenia Gravis

Inclusion Criteria: * The participant (and/or their legally authorized representative) understands the requirements of the study and is capable of providing written informed consent/assent and complying with protocol requirements * The participant is aged 2 to \<18 years at the time of informed consent/assent * The participant has been diagnosed with generalised Myasthenia Gravis that is supported by a physical examination and confirmed seropositivity for anti-acetylcholine receptor antibodies * The participant has had an unsatisfactory response to immunosuppressants, corticosteroids, or acetylcholinesterase inhibitors but is on stable concomitant MG therapy. If receiving corticosteroids and/or immunosuppressants, must be on a stable dose for ≥1 month before screening * The participant agrees to use birth control consistent with local regulations and people of child-bearing potential must have a negative blood pregnancy test at screening and a negative urine pregnancy test before receiving the study drug Exclusion Criteria: * Is a female adolescent of child-bearing potential who is pregnant and/or lactating or intends to become pregnant during their participation in the study * Has worsening muscle weakness secondary to a concurrent infection or as a result of a medication * Has a documented lack of clinical response to plasma exchange (PLEX) * Received a live or live-attenuated vaccine within \<4 weeks before screening * Received a thymectomy within 3 months before screening or is planning to get a thymectomy during their participation in the study * Has a known autoimmune disease or any medical condition that would interfere with an accurate assessment of clinical symptoms of generalised Myasthenia Gravis or puts the participant at undue risk * History of malignancy, cancer, unless considered cured by adequate treatment with no evidence of recurrence for ≥3 years. Adequately treated participants with the following cancers can be included at any time: 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 * Clinically significant active infection that is not sufficiently resolved in the investigator's opinion or positive serum test at screening for active infection with any of the following: Hepatitis B virus (HBV), Hepatitis C virus (HCV), HIV * Has a positive PCR test for SARS-CoV-2 at screening * Has/had a clinically significant disease, had recent major surgery (within 3 months of screening) or intends to have major surgery during the study, or has/had any other medical condition that, in the investigator's opinion, would confound the results of the study or put the participant at undue risk * Has received a different study drug in another clinical study within \<12 before screening * Is currently participating in another interventional clinical study * Has previously participated in an efgartigimod clinical study and received at least one dose of study drug * Has a known hypersensitivity to study drug or any of its excipients * Has a history of or current episode of alcohol, drug, or medication abuse as assessed by the investigator * Use of some medications before screening (more information is found in the protocol) The complete list of exclusion criteria can be found in the protocol.