Interpretation of ileal biopsies: morphological features in normal and diseased mucosa

One of the advantages of flexible colonoscopy is that the terminal ileum can easily be reached and biopsied by an experienced operator. Thus, the pathologist will, with increasing frequency, receive ileal biopsies from patients who have or may not have ileal disease and it is therefore useful to have a good knowledge of the spectrum of ileal mucosal histology and pathology, of its normal function, and of the changes seen in different conditions. It is also desirable to be aware of diseases that occur less frequently. The ileal microscopic features are related to its absorptive function on the one hand, and to non-specific and specific defence mechanisms of the organism against potential hazardous components on the other. As a consequence the mucosa features a constant physiological and controlled inflammatory process. Part of the ileal mucosal structure is determined by the presence of the gut-associated lymphoid tissue (GALT) which plays a key role in discriminating harmless nutrients and harmful pathogens. It will be important to distinguish this normal situation from alterations seen in infectious or inflammatory pathology. Further, because of the therapeutic implications, it is necessary that acute and chronic (idiopathic) inflammation are recognized. This will in general be possible by evaluating the composition of the inflammatory infiltrate and the occurrence of epithelial and mucosal architectural changes.