also called Reticuloendothelial System or Macrophage System
The lymphoreticular system consists of the spleen, lymphnodes, lymphatic vessels, thymus, and bone marrow. The functions of these systems include immune defense, transport of fats throughout the body, and collection and transport of interstitial fluid (the fluid bathing the cells) back to the circulatory system.
The spleen is an important lymphoreticular organ that functions in immune defense. Unlike the spleen of pinnipeds, the spleen in cetaceans is much smaller and rounded, often with several “accessory” spleens nearby. When opening up the abdomen of a porpoise the spleen is usually hidden behind the stomachs.
The thymus is a lobular gland located cranial to the heart and caudal to the hyoid apparatus. The thymus changes in size dramatically with ontogeny, starting off very large in young animals and slowly shrinking with age. The thymus is involved in “training” the T-cells of the immune system early in life.
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.
"Reticuloendothelial system" is an older term for the mononuclear phagocyte system, but it is used less commonly now, as it is understood that most endothelial cells are not macrophages.