In the last few decades, new immunosuppressive drugs have improved our ability to treat autoimmune diseases and perform successful transplantation procedures. Despite the success of these drugs, serious side effects including generalized immunosuppression, infections, cancers, diabetes and seizures considerably decrease patients’ quality of life. Recent research in this area has focussed on T regulatory (Tr) cells, a recently characterized subset of white blood cells that have the ability to suppress undesired immune responses, while leaving other aspects of the normal immune system intact. Sarah Allan’s research addresses questions about the molecular and cellular biology of Tr cells. She is investigating how Tr cells arise naturally, the mechanisms by which they suppress immune responses, and how they differ from other types of T cells at the molecular and genetic level. Ultimately, her work will contribute to the development of novel therapies for autoimmune diseases, transplantation, and other pathologies of the immune system.