The liver: a model of organ-specific lymphocyte recruitment.

The liver is constantly exposed to gut-derived antigens that enter via the portal vein, and it must modulate immune responses so that harmful pathogens are cleared but necessary food antigens are ignored. The liver contains a large resident and migratory population of lymphocytes and macrophages that provide immune surveillance against foreign antigen. This population of cells can be rapidly expanded in response to infection or injury by recruiting leukocytes from the circulation, a process that is dependent on the ability of lymphocytes to recognise, bind to and migrate across the endothelial cells that line the vasculature. Lymphocytes can enter the liver at several sites: the vascular endothelium in the portal tracts (comprising the hepatic artery, portal vein and bile ductule), the sinusoids (through which the blood percolates past the hepatocytes) or the central hepatic veins (through which the blood exits). The requirements and physical conditions at each site vary and there is evidence that different combinations of adhesion proteins are involved at these different sites. This article discusses the expression and function of adhesion molecules within the liver and demonstrates how specific populations of effector lymphocytes can be selectively recruited to the liver.