Immunoglobulin Deficiency a Treatable Cause of Fatigue in Patients With Multiple Sclerosis (MS)?

Inclusion Criteria: * Diagnosis of Multiple Sclerosis following McDonald 2017-Criteria * Age 18-65 years * Stable MS disease at inclusion (definition: no clinical relapse, no MRI activity, stable disability within the last 12 months) * Unchanged immunotherapy within the last 12 months * Expanded Disability Status Scale (EDSS) level \<4 points indicating fully ambulatory patients. * Capability of written informed consent Exclusion Criteria: * Severe depression (definition: Beck Depression Index-II (BDI-II) ≥29 points) or other established psychiatric diagnosis * Immunodeficiency other than hypogammaglobulinemia * Immunglobulin replacement therapy or indication for immunoglobulin replacement therapy * Severe Sleepiness (definition: Epworth-Sleepiness-Scale (ESS) \>16 points) * Fatigue aggravating factors such: liver/renal/thyroid dysfunction, substance abuse, medication (tranquilizers /antiepileptics/psychopharmaceuticals), chronic infectious disease (like hepatitis/HIV). * Other neurodegenerative/autoimmune disease. * Patients not able to give written consent * Vulnerable patients such as children, pregnant women and prisoners