Study to Assess Effects of Ublituximab in Pediatric Participants With Relapsing Forms of Multiple Sclerosis

Inclusion Criteria for Part A and Part B: 1. Diagnosis of RMS. 2. EDSS at screening: 0-5.5, inclusive. 3. Neurologic stability for ≥ 30 days prior to screening, and between screening and Week 1 Day 1 (W1D1). Inclusion Criteria for Part C: 1\. Participants must have completed Part A (Week 24 visit) or Part B (Week 96 visit) to be eligible for Part C. Exclusion Criteria for Part A and B: 1. Known presence or suspicion of other neurologic disorders that may mimic MS. 2. Prior treatments: 1. Systemic corticosteroids (\>0.1 milligrams/kilogram/day \[mg/kg/day\], or \>5 milligrams/day \[mg/day\] of prednisone equivalent) or adrenocorticotropic hormone (ACTH) within 30 days prior to the screening MRI scan (note: Topical, ophthalmic, or inhaled corticosteroids are permitted). 2. High dose intravenous immunoglobulin (IVIG) or subcutaneous IG (SCIG) within 2 months prior to W1D1. 3. Treatment with anti-CD20 or other B cell directed treatment at any time. 4. Treatment with alemtuzumab, cladribine, cyclophosphamide, mitoxantrone at any time. Additional Exclusion Criteria for Part B Only (Relevant to Fingolimod Treatment): 1. Treatment with fingolimod or other sphingosine-1 phosphate-1 (S1P1) modulators at any time. 2. The following antiarrhythmic drugs at Screening: Class Ia anti-arrhythmics. Exclusion Criteria for Part C: 1\. If the absolute lymphocyte count (ALC) is outside the specified range the participant will not be eligible to receive ublituximab in Part C. Note: Other protocol-specified inclusion/exclusion criteria may apply