Numerous immune abnormalities have been described in multiple sclerosis, including dysfunction of humoral, cellular and non specific immunity. The main abnormality probably concerns suppressive T cell function. The decrease of this function could explain the breakdown of immunological tolerance towards myelin antigens. The primary responsibility of these abnormalities in the triggering of the disease has still to be demonstrated. Moreover, the antigenic target of immune dysfunction has not yet been determined. A better understanding of T cell traffic into the central nervous system and of the mechanisms of interactions between the T cells and the antigen on one hand, the T cells and the cells presenting antigen on the other hand, offer encouraging therapeutic possibilities.