The liver is the largest solid organ in the body and has many unique immunological properties, including induction of immune tolerance, strong innate immunity, poor adaptive immune response versus overreactive autoimmunity and hematopoiesis in the fetal liver. Thus, the liver has been proposed as ‘an immunological organ’. 1 Although the primary functions of the liver are not traditionally considered to be immunological, the liver also performs many essential immune tasks. For example, hepatocytes are responsible for the production of 80–90% of the circulating innate immunity proteins in the body, and the liver contains a large number of resident immune cells. Therefore, researchers have also proposed that the liver is ‘an innate immune organ’, 2 ‘an immune organ’3 and ‘a lymphoid organ’. 4, 5 I am honored and excited to present this special issue that contains eight excellent review articles about the current knowledge of these fascinating immunological features of the liver. The first article by Robinson et al. 6 provides an overview of liver immunology and its role in liver inflammation and homeostasis. The following two papers discuss the special properties of antigen-presenting cells in the liver and their contribution to the hepatic tolerogenic effect. 7, 8 The last six articles explore hepatic innate immunity 9–13 and adaptive immunity. 14

Liver immune tolerance was first observed in porcine liver allo-transplantation in the 1960s and was later found to be indispensable for the maintenance of oral tolerance toward food and bacterial antigens from the gut. Over the past several decades, the immune tolerogenic function of the liver has been extensively investigated by many groups, but the underlying mechanisms still remain obscure. Evidence suggests that livermediated immune tolerance requires the complex interaction of hepatocytes, liver nonparenchymal cells and immune cells. In this special issue, two articles discuss the potential cellular and molecular mechanisms through which immune tolerance occurs in the liver. Horst et al. 7 summarize the characteristics of several types of conventional and nonconventional antigen-presenting cells, as well as regulatory innate immune cells in the liver, which may have an important role in inducing immune tolerance. Grakoui and Crispe 8 also summarize many unique features of antigen-presenting cells that present hepatocellular antigens in the liver and discuss their potential role in mediating the hepatic tolerogenic effect. The liver is constantly exposed to a wide variety of bacterial products, environment toxins and food antigens. To efficiently and rapidly protect against these potentially toxic agents without generating detrimental immune responses, the liver depends on its strong innate immune system, acting as an important innate immunity organ. 2 In this special issue, five articles update and discuss the innate immunity in the liver. First, Zhou et al. 9 provide evidence supporting the role