Kupffer cells, also known as stellate macrophages and Kupffer–Browicz cells, are specialized cells localized in the liver within the lumen of the liver sinusoids and are adhesive to their endothelial cells which make up the blood vessel walls. Kupffer cells comprise the largest population of tissue-resident macrophages in the body. Gut bacteria, bacterial endotoxins, and microbial debris transported to the liver from the gastrointestinal tract via the portal vein will first come in contact with Kupffer cells, the first immune cells in the liver. It is because of this that any change to Kupffer cell functions can be connected to various liver diseases such as alcoholic liver disease, viral hepatitis, intrahepatic cholestasis, steatohepatitis, activation or rejection of the liver during liver transplantation and liver fibrosis. They form part of the mononuclear phagocyte system.