BURNET’S THEORY REVISITED Nearly half a century ago, Burnet theorized that the body learns to distinguish between self and non-self. It seemed that the foundation for this distinction and for self tolerance gets set at early stages of life. Now three independent groups, have reexamined this concept. Ridge et al in the March 22, 96 issue of Science report that both neonatal and adult T cells behave in a similar fashion when exposed to antigens. Both types of T cells can be activated by professional antigen presenting cells and both can be tolerized by non-costimulatory cells. For tolerization, the main determinant for both types of cells is the dose of the antigen. Both the T cells of neonates and adult cells can be tolerized. The findings point to the fact that it takes far fewer number of cells to generate tolerance in the newborn than in adult animals. In addition to these findings, the data from Sarzotti et al and Forsthuber et al in the same issue of Science show that the immune system of the neonates is quite capable of mounting an immune response under the right conditions. Perhaps the most intriguing aspect of these findings is the capability to induce tolerance in adult animals. If such findings are applicable to humans, then one can prevent the rejection of transplants, and devise strategies for the treatment of autoimmune disorders.