Significance Multiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS). B cells play a key role in MS immunopathology, as demonstrated by the success of B cell-directed therapies; however, the target antigen of MS remains unknown. Using a combination of ELISpot-based prescreening of peripheral blood mononuclear cells followed by investigation of antibody specificity with a CNS antigen array, we identified a population of MS patients characterized by a highly active B cell response. These individuals with MS, who have active B cell responses, exhibited heterogeneous interindividual anti-CNS antibody responses, although the antigenic specificity of the antibodies remained stable over time for each individual.