About half of the people with multiple sclerosis have the HLA-DR15 gene variant. A study led by the University of Zurich has now shown how this genetic predisposition contributes to the development of the autoimmune disease multiple sclerosis in combination with environmental factors. The decisive factor is the shaping of a repertoire of immune cells which – although they are effective in fighting off pathogens such as Epstein-Barr virus – also attack brain tissue.