In essence bone marrow stem cells establish chimaerism of the host. Lifelong chimaerism is needed to maintain tolerance and even a low level of chimaerism is sufficient. For the first few days of the mouse's life there are too few post-thymic T cells to sustain a response and these are tolerised either by peripheral deletion or some other mechanism. If the transfer is done later the number and maturity of the peripheral T cell pool is sufficient to destroy the donor stem cells before they can engraft. Even antigens which cannot establish chimaerism can establish a state of tolerance in neonatal mice, this is less permanent however.
Timing is an important parameter on the tolerisability of the immune system.
If we could induce tolerance in adults, this would have obvious benefits in transplantation and the treatment of autoimmunity. Much effort has therefore been expended towards this goal. Indeed a number of regimes have been successfully applied under experimental conditions and with particular antigens. A clinically usable protocol is still elusive however. It will suffice here to give just two examples of tolerising conditions which have worked.