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

The primary purpose of this study is to evaluate the pharmacokinetics (PK) and pharmacodynamics (PD) of ublituximab in participants ages 10 to less than (\

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