[Some aspects of phylogenetic development of cellular immune reaction in vertebrates]

The division of the immune system into humoral and cell-mediated immunity is established in vertebrates. Moreover single manifestations of cell-mediated immunity are observed in invertebrates, too. In protochordates, e.g., a genetically fixed ability to recognize "Self" and "Not-Self" could already be demonstrated, but the interpretation of this phenomenon as a preliminary stage of cell-mediated immunity in vertebrates is not established yet. Especially by the investigation of the transplantation tolerance, for example between mammalia and inferior vertebrates, essential results as to the development of the immune system in vertebrates could be obtained. It is quite sure that the experimental findings obtained till now are not sufficient for giming a complete picture of the phylogenesis of immunologic reactivity. But they very unanimously support the conception that nearly all of the nowadays living representatives of the vertebrates dispose of cell-mediated immunity, which is equal in its characteristic features. And the small lymphocyte takes the central part in the cell-mediated immune reactions of the vertebrates, and the complexity of the lymphoid organs, which is increasing in the course of phylogenesis leads to a quantitative but not to a qualitative change.