This clinical study is testing a drug called **efgartigimod** to see if it's safe and helps people with a condition called **Generalized Myasthenia Gravis (gMG)**. gMG is a disease that makes muscles weak and tired easily. Some people with gMG have a special antibody that helps doctors diagnose them, but this study is for people who do not have that antibody (seronegative).

In the study, participants will receive efgartigimod or a placebo (a substance with no active drug) through an **intravenous (IV)** line, which means directly into the vein. The study has two parts: Part A compares efgartigimod with placebo, and Part B gives the real drug to all participants.

• Participants must be at least 18 years old.
• The study involves IV treatments and will last several weeks.
• Participants cannot join if they have certain medical conditions or are pregnant.

Before joining, participants will be screened to ensure they meet the requirements. Those with stable gMG treatment, but without certain health issues, are eligible. The study is important for finding better treatments for people with seronegative gMG.