cosal injury ( Figure 1B) ? Mucosal surfaces, such as the intestinal mucosa and the upper respiratory tract, are constantly exposed to environmental stimuli, such as commensal luminal bacteria in the intestine, as well as endogenous stimuli, or, as is the case in the lower respiratory tract and urinary tract mucosa, may encounter and respond to endogenous and exogenous stimuli in disease states. In either case, the mucosa must be able to mount an effective immune response, resist barrier failure, and coordinate this response with both mucosal and sub-mucosal leukocytes, while avoiding initiation and propagation of an exaggerated inflammatory response. But where does the answer lie in terms of what is initiating the mucosal inflammatory response?