Aging of the immune system occurs in aged individuals and in a proportion of patients with multiple sclerosis (MS). One of the features of immune aging is the expansion of aged B cells with adverse functional characteristics. In a pilot study, we have found an increased number of aged B cells in the peripheral blood of a proportion of MS patients. We now aim to further study whether aged B cells are increased in the peripheral blood, cerebrospinal fluid and brain tissue of MS patients. To unravel the contribution of aged B cells to MS pathology, we will investigate the mechanisms underlying their development in MS and their functional properties.