Introduction

The gastrointestinal mucosal immune system contains a large, complex mixture of cells that play an essential role in host defence against pathogens and in maintaining the normal balance of tolerance and immunity to constituents of the gastrointestinal lumen. Cells are organized in specialized structures: Peyer's patches (which are important in initiating immune responses and in non-organized compartments) and the lamina propria, where specialized, differentiated cells carry out effector functions. Many of the specialized functions of cells in this system depend on the release of cytokines and other mediators. Recent studies have demonstrated the importance of the gastrointestinal flora in modulating the activity of the mucosal immune cells and exacerbating gastrointestinal inflammation. The function of gastrointestinal immune cells and their mediators appear to be altered in inflammatory bowel diseases and offer potential targets for pharmacological intervention.