The intestine has a surface area nearly two hundred times greater than the skin, and is exposed to many pathogenic micro-organisms. It requires an effective immune response to protect it from infections. On the other hand, immune responses must not be made against the harmless bacteria and food proteins that are also present. When they occur, these inappropriate responses have serious consequences, leading to inflammatory bowel diseases or food allergies. To ensure our health, the balance between ?immunity? and ?tolerance? must be maintained. A critical cell for maintaining this balance is the dendritic cell (DC). DCs migrate continually, in lymph, carrying information about the intestine to the immune cells in the mesenteric lymph nodes (MLN). On reaching the MLN, DCs interact with T lymphocytes. This interaction is thought to control whether immunity or tolerance will occur. Understanding how DCs control these processes is likely to be highly beneficial in the design of oral vaccines or prevention of inflammatory bowel diseases. However, study of migrating DCs has been difficult because they cannot easily be separated from other cells in the MLNs. The only way to collect cells which are certain to be migrating DCs is by surgery, which is used to collect the DC-containing lymph. Previously this surgery has only been possible in large animals. We have developed humane surgical techniques to collect migrating DCs from mice, and to return them to the MLNs. This will enable us, for the first time, to study these cells and their interactions with T cells, using the sophisticated immunological techniques and resources that are only available in mice. We are currently the only group in the world able to collect migrating DCs from mouse lymph. We believe that the study of these cells will generate important information about how immune responses are controlled.