A contemporary view of transplantation tolerance: an immunologist's perspective

The unique feature of the immune system is its capacity to distinguish between self and non-self components. The selectionists have postulated that deletion of clones during development could account for this phenomenon. The Bretscher/Cohn theory of self/non-self discrimination provided a model whereby self-reactive clones could be eliminated at any stage of the animal's life. Experimental evidence demonstrates that clonal deletion can occur and is probably dependent on lympho-hematopoietic chimerism involving the passage of cells to the thymus. However, tolerance to peripheral tissue antigens that do not have direct access to the thymus is not maintained by deletional tolerance. In this situation an active process negatively regulates the response of the immune system to non lymphoid, tissue-specific antigens.