 However, later studies showed that cells or tissues transplanted before the immune system development of the recipient, such as in embryonic recipients, could be treated as foreign and trigger rejection, countering Burnet's explanation for self tolerance. In contrast to the Burnet hypothesis of a special tolerance-inducing period defined by the age of the animal, Joshua Lederberg proposed in 1959, that it is the age of the lymphocyte that defines whether an antigen that is encountered will induce tolerance, with immature lymphocytes being tolerance-sensitive. Lederberg's concept is now known as central tolerance, and is widely accepted. It may also explain the success of some transplants given early in life and the failure to induce tolerance in other studies. Burnet noted that his contributions to immune tolerance were strictly theoretical:
