Multiple sclerosis is an inflammatory disease of the central nervous system responsible for demyelinating lesions within white matter in young adults. The activation of autoreactive CD4+ but also CD8+ T cells directed against myelin antigens of the central nervous system is a key phenomenon at the origin of multiple sclerosis lesions. Its etiology is unknown but implies the existence of genetic traits and a triggering by environmental factors such as viral infections. However, no virus or other environmental factor has been directly involved in the triggering of the disease. Functionnal abnormalities on regulation systems of immunity, such as regulatory T cells, could be partially involved in the triggering of the disease. However, it appears, on the basis of recent findings, that multiple sclerosis is not only a disease of the immune system but also implies specific neurobiological factors which must be determined in studies to come.