Autoimmune diseases are subsequent to tissue damage mediated by an immune reaction directed toward autoantigens. Activation of CD4+ T cells reactive to carrier determinants on foreign antigenic complexes or autoreactive CD4+ T cells are central to the triggering of immune effector mechanisms. Transfer experiments in animals, demonstration of suppressor T cells, study of antigen-presenting cells–CD4+ T cells interaction, isolation of T cell clones and study of transgenic mice have recently brought new insight into the understanding of autoimmunity.