A Study of Nipocalimab Administered to Adults With Generalized Myasthenia Gravis

The purpose of this study is to evaluate the efficacy and safety of nipocalimab compared to placebo in participants with generalized myasthenia gravis (gMG). The purpose of the subcutaneous substudy is to evaluate how well it works in the body (pharmacodynamic \[PD\]) when given as an injection under the skin (subcutaneous) compared to when given through a vein (intravenous) in participants with gMG.

Inclusion Criteria: * Diagnosis of myasthenia gravis (MG) with generalized muscle weakness meeting the clinical criteria for generalized myasthenia gravis (gMG) as defined by the Myasthenia Gravis Foundation of America (MGFA) Clinical Classification Class II a/b, III a/b, or IVa/b at screening * Myasthenia Gravis - Activities of Daily Living (MG-ADL) score of greater than or equal to (\>=) 6 at screening and baseline * Has sufficient venous access to allow drug administration by infusion and blood sampling as per the protocol * A woman of childbearing potential must have a negative highly sensitive serum (beta-human chorionic gonadotropin \[beta-hCG\]) at screening and a negative urine pregnancy test at Day 1 prior to administration of study intervention * A male participant must agree not to donate sperm for the purpose of reproduction during the study and for a minimum 90 days after receiving the last administration of study intervention * For the SC Substudy (Cohort 1 and Cohort 2): Has reasonable abdominal skin area for SC administration * For the SC Substudy (Cohort 1 and Cohort 2): Participants must be willing to comply with maintaining their stable dose of corticosteroids and/or immunosuppressants for the initial 8 weeks of the SC substudy, that is, through the SC Week 8 visit Exclusion Criteria: * Has any confirmed or suspected clinical immunodeficiency syndrome not related to treatment of his/her gMG, or has a family history of congenital or hereditary immunodeficiency unless confirmed absent in the participant * Has MGFA Class I disease or presence of MG crisis (MGFA Class V) at screening, history of MG crisis within 1 month of screening, or fixed weakness (and/or 'burnt out' MG) * Has had a thymectomy within 12 months prior to screening, or thymectomy is planned during the study * Has known allergies, hypersensitivity, or intolerance to nipocalimab or its excipients * Has experienced myocardial infarction, unstable ischemic heart disease, or stroke within 12 weeks of screening * For the SC Substudy (Cohort 1): Participants who have undergone a recent tapering of their concomitant MG medication in the OLE * For the SC Substudy (Cohort 1): Participants deteriorating during the OLE in the month prior to SC Dose 1 of the SC substudy such that they meet the criteria for clinical deterioration * For the SC Substudy (Cohort 2): History of an unprovoked pulmonary embolism within 1 year prior to screening or history of recurrent deep vein thrombosis (DVT)