The mononuclear phagocyte system (MPS) (also called Reticuloendothelial System or Macrophage System) is a part of the immune system that consists of the phagocytic cells located in reticular connective tissue. The cells are primarily monocytes and macrophages, and they accumulate in lymph nodes and the spleen. The Kupffer cells of the liver and tissue histiocytes are also part of the MPS. Mononuclear phagocyte system and monocyte macrophage system are two different terms, often mistakenly understood as one.