IV. Lymphocyte trafficking in the intestine and liver.

Naive lymphocytes patrol continuously between the blood and different lymphatic tissues to sample the whole body for foreign antigens. During inflammation, leukocyte recruitment into tissue is enhanced to promote the recruitment of a range of effector cells into the affected area. The complex recirculatory pathways that underlie this process are governed by adhesion receptors on blood-borne leukocytes and by their specific ligands expressed on the luminal aspect of endothelial cells lining the vessels. Gut-associated lymphatic tissues are positioned strategically at the major port of entry for foreign antigens. They form a functionally unified entity that utilizes both the afferent and efferent arms of the immune response to respond to the large array of antigens entering via the gut under normal conditions as well as during inflammation. Once antigens have been absorbed from the gut, they may enter the portal vein and the liver where the immune response can be further regulated by the resident immune cells of the liver. Thus the gut and liver form an important barrier to enteral antigens, and leukocyte recruitment to these sites will need to be carefully regulated to ensure effective immune surveillance. In this article, we describe the current concepts of lymphocyte adhesion in these two organs as revealed by animal models. Subsequently, we discuss how well these principles apply to the lymphocyte-endothelial cell interactions in humans and what additional insights can be obtained from human studies.