Specialized commensal bacteria occupy niches in different compartments of the GIT. The structure of the murine intestine in different parts of the GIT most certainly reflects its dominant purpose of digesting and absorbing nutrients. However, the presence and usage of various immune-response mechanisms also mirror the interplay with the respective resident intestinal bacteria in compartments that these bacteria are adapted to colonize as a permanent or temporary ecological niche. The immune response, in return, can shape the commensal bacterial flora (15). Whereas the total bacterial load increases toward the lower GIT, distinct bacteria are adapted to predominantly colonize niches in the small intestine or cecum (refs. 1, 14; and M. Strus et al., unpublished observations). Original magnification of the micrographs, ×200. spp., species.