Watch out gastroenterologists! Many immunologists are now turning their attention to the study of immune responses at mucosal sites. Mucosal immunology is attractive for several reasons. Mucosal surfaces comprise the body's first line of defence against numerous pathogenic organisms. And understanding how the mucosal immune system is typically able to distinguish pathogens from the commensal microflora and innocuous food antigens and respond appropriately to each challenge is a fascinating conundrum. When the mucosal immune system fails to discriminate between good and bad, the consequences — such as inflammatory bowel disease and food allergy — can be devastating. Exciting research is now providing clues to the inner workings of the mucosal immune system, and the harnessing of its unusual features to prevent or treat disease shows promise for the development of new mucosal vaccines, adjuvants and immunotherapeutics.

Five articles in this issue review the latest developments in the field of mucosal immunology. David Artis assigns renewed importance to the functions of intestinal epithelial cells. Janine Coombes and Fiona Powrie dissect the role of dendritic cells in intestinal immune regulation. Andrea Cerutti details the regulation of IgA, the body's most abundant antibody class. Judy Cho draws on lessons learnt from genome studies to understand inflammatory bowel diseases, and Florian Hladik and Juliana McElrath cover the invasion of genital mucosal sites by HIV.

The importance of this fast-growing field is also illustrated by the recent partnership between the Society for Mucosal Immunology and the Nature Publishing Group in the production of the new journal Mucosal Immunology. Finally, we thank the Federation of Clinical Immunology Societies (FOCIS) and Beckman Coulter for supporting this Focus issue.