The Liver as a Lymphoid Organ

The liver is the largest solid organ in the body that not only has metabolic and detoxifying functions but also serves as a unique lymphoid organ. It continuously receives antigens from the blood circulation, including diet and commensal microbiota antigens, as well as toxic chemicals and apoptotic cells. Thus, the liver contains a predominant proportion of cells with innate immune functions that can react quickly against pathogens and also can preferentially induce and participate in immune tolerance. Both non-hematopoietic cells and immune cells in the liver participate in local and systemic immune responses. In this chapter, we summarize basic aspects of liver immunology that make the liver a “lymphoid organ,” with an emphasis on the heterogeneity of immune cell subsets and their roles in immune homeostasis and pathology. We also discuss how the microenvironment regulates immune cell composition, location and function within the liver, as well as the role of hematopoiesis in the adult liver.