[The self and its enemy].

This essay uses the form of an imaginary debate between an immunologist and a non-immunologist to discuss the problem of immunological self-recognition and the clinical relevance of this phenomenon to the development of autoimmune disease. The first part deals with the communication between different cell types involved in normal and abnormal immune reactions. Then the problems of "physiological" autoimmunity and regulation of the normal immune response are discussed. The development of autoimmune diseases is reviewed on the basis of data obtained in the so-called Obese strain (OS) of chickens, which show a spontaneous hereditary autoimmune thyroiditis analogous to human Hashimoto thyroiditis. The role played by the thymus for the homeostasis of a normal immunological reactivity is emphasized. Finally, parallels are drawn between those mechanisms underlying the development of autoimmune disease and changes in the normal immune system with increasing age. The contribution finishes with some thoughts on the potential applications of our present knowledge of the process of auto-immunity to new therapeutic approaches to this large group of human diseases.