1. Adult patients (≥ 18 years old) who had been treated with ocrelizumab for a relapsing form of Multiple Sclerosis according to the ocrelizumab marketing authorization (MA) treatment regimen for at least 18 months.
2. Patients with a decrease of IgG at a minimum of two consecutive dosages (around six months apart) during ocrelizumab treatment\*.
3. Patients already switched or switching from ocrelizumab to ofatumumab treatment (decision independent from the study\*).
4. Patients agreeing to participate to the study.
5. Able to provide their non-opposition
* At the discretion of the investigator, if a benefit for patients is expected based on their experience and depending on patient's characteristics
You may not be if
1. Patients with ocrelizumab dose spacing \> 8 months in the last 18 months before switch
2. Patient who underwent an immunoglobulin supplementation therapy within 18 months before the switch
