Scientific interest in regulatory T cells has revived during the last decade. Initially described in the early seventies as suppressor T cells, the concept of suppressor/regulatory T cells went through turbulent times during the eighties when molecular analysis failed to identify putative suppressor genes. The constructive and elegant cellular experiments on regulatory T cells during the nineties, initiated by Shimon Sakaguchi and co-workers, however have brought these cells back into the limelight. Nowadays, regulatory T cells are regarded as essential components of the immune system, and...