Evaluating Long-term Safety of Efgartigimod Administered Intravenously and Efgartigimod PH20 Administered Subcutaneously in Children with Generalized Myasthenia Gravis

Inclusion Criteria:

Participants are eligible to be included in the trial only if all of the following criteria apply:

1. The participant completed ARGX-113-2006 or ARGX-113-2207, defined as:

2. The participant reached End of Trial in trial ARGX-113-2006 or End of Study inARGX-113-2207 and agreed to participate in the ARGX-113-2008 trial.

3. The participant qualifies for retreatment in trial ARGX-113-2006, but cannotcomplete a Treatment Period (TP) and the required Intertreatment Period (IP)visits within the ARGX-113-2006 trial's timeframe.

4. Either the participant or the participant's legally authorized representative canunderstand the requirements of the trial and provide written informedconsent/assent, and willingness and ability to comply with the trial protocolprocedures.

5. Contraceptive use for sexually active participants should be consistent with localregulations for those participating in clinical studies. A participant is ofchildbearing potential if, in the opinion of the investigator, he/she isbiologically capable of having children.

6. Contraceptive requirements for male participants are presented in Section 10.4.2.2.

7. Female adolescents of childbearing potential (FAOCBP) must have a negativeurine pregnancy test at trial entry (TP1V1 or IP0V1)

Exclusion Criteria:

Participants are excluded from the trial if any of the following criteria apply:

1. Female adolescents of childbearing potential (FAOCBP): Pregnancy or lactation, orthe participant intends to become pregnant during their participation in the study.

2. Discontinued early from ARGX-113-2006 or ARGX-113-2207 treatment.

3. A known hypersensitivity reaction to efgartigimod or any of its excipients.

4. Any of the following medical conditions:

5. Clinically significant uncontrolled chronic bacterial, viral, or fungalinfection at study entry not sufficiently resolved in the investigator'sopinion.

6. Known autoimmune disease or any medical condition that would interfere with anaccurate assessment of clinical symptoms of generalized myasthenia gravis (gMG)or put the participant at undue risk