The importance of the dendritic cell for the capture of antigens and initiating an immune response is now well recognized. Whereas much is known about their structure and function, their lineage is still not clear. Studies in vitro have demonstrated that the regulated maturation of function that occurs in culture explains many of the in vivo events relating to antigen capture and presentation. The control over maturation and migration of these cells to the immune system is decisive as to whether an immune response is mounted or not. 'Danger' signals provided by conserved bacterial products or by microenvironmental cytokines are important regulators. Dendritic cells have been clearly involved in the development of respiratory disease and our understanding of their involvement will have an impact on our future therapeutic strategies.