Liver macrophages: development, dynamics, and functions

The liver is a sizeable visceral organ whose primary functions involve nutrient metabolism, clearance of toxins, and energy storage. Besides these critical functions, the liver is also a major immunological site. It is populated by several specialized resident immune cells, including B cells, T Cells, dendritic cells, and several populations of macrophages. It is also the site for the production and release of acute-phase proteins during inflammation. One reason for garrisoning these immune sentinels and effectors in the liver is its relative location in the circulatory system. The liver is the first significant organ downstream of the intestine, where blood originating from the intestine enters the liver through the portal vein. This organization facilitates the liver's uptake and processing of nutrient-rich blood directly from the intestinal source. However, the intestine is also home to trillions of microbes, many of which are commensals but also represent potential pathogens. As such, the portal blood supply represents an avenue for systemic infection. To sterilize the portal blood, the liver immune system filters pathogens, which is primarily accomplished by liver macrophages. Here, we will discuss the major populations of macrophages resident in the liver, their location, functions, development, and role in maintaining the liver in the face of injury and infection.