Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system involving autoimmune mechanisms. MS has been treated as a disorder mediated mainly by T cells for a long time. However, recent findings demonstrate that B lymphocytes are of crucial pathogenetic significance in MS. In patients with MS, B cells can possess both pro-inflammatory and anti-inflammatory effects. The paper reports the main mechanisms of B lymphocyte involvement in the pathogenesis of MS. Diagnostic value of assessing humoral immune parameters in individuals with demyelinating diseases and modern possibility of B cell function modulation are discussed.