The immune system tries to maintain an optimal balance between immune responses to control infection and tumour growth, and reciprocal responses that prevent inflammation and autoimmune diseases. Impaired immune responses, such as those that occur with autoimmune disorders (multiple sclerosis, type 1 diabetes) and organ transplant rejection, result when a person’s immune system mistakenly attacks normal cells. Currently, patients afflicted with this condition must follow a strict regime of immunosuppressive drugs for the rest of their lives. However, these treatments seriously compromise the body’s ability to fight infection and also increase the risk of developing cancer. Sarah Crome is studying the role of a newly discovered class of cells, called T regulatory (Treg) cells in immune system response. She is studying how Treg cells suppress other immune cells and essentially act as a “brake” for the immune system. She is also examining how a subset of T cells, called T helper 17 cells, cause harmful immune responses that result in the rejection of transplanted tissues. A better understanding of these cells and the interactions and factors that regulate their differentiation and function, may lead to more effective treatments for organ transplantation and autoimmune diseases without compromising normal immune function.