Gastrointestinal immunology: cell types in the lamina propria--a morphological review

Defense mechanisms--including immune responses--of the gastrointestinal (GI) system rely on a delicate balance of multidirectional interactions of different components of the GI mucosa. The majority of the cells involved in immune reactions are in the lamina propria (LP) and in the submucosa. Several biologically active substances (enzymes, neurotransmitters, humoral mediators) and their receptors have been reported to be present in LP cells. These cells are in close morphological connections with the surface epithelial cells and with the surrounding vessels and nerve fibers, suggesting a functional association with them. In this paper cell types of the LP will be reviewed from a morphological aspect. In summary, LP cells can be classified as basic structural elements (fibroblasts, fibrocytes, vascular endothelial and smooth muscle cells), blood cells (granulocytes, mast cells, macrophages, T and B lymphocytes, plasma cells), and occasional epithelial and endocrine cells of the surface epithelium. Nerve fibers and terminals, but not neuronal perikarya, can also be seen in the LP. The appearance and the proportion of LP cells strongly depend on the functional activity of the GI system at any given time. Their number and distribution might be significantly altered in certain pathological conditions (infections, inflammations, ulcerations, and other GI disorders). We hope, this review may help clinicians, pathologists, and researchers in the recognition of LP cell types, and in demonstrating their activation, migration and proliferation in different physiological and pathological conditions.