Monocyte-Macrophage System (Reticuloendothelial System). Chapter 34 part 4. Guyton and Hall Physio.

The monocyte-macrophage system, or reticuloendothelial system, comprises all monocytes and macrophages in the body, found in areas like the skin, lungs, GI tract, lymph, and blood. Monocytes in the blood transform into tissue macrophages (mobile or fixed) upon entering tissues. Specialized endothelial cells in lymph nodes, bone marrow, and spleen also house collections of macrophages.

The primary function of monocytes and macrophages is defense against foreign invaders. They are strategically located at common entry points for pathogens. In the skin, they are called histiocytes. In the lungs, lung macrophages engulf foreign organisms. If a foreign body is indigestible, such as tuberculosis bacteria or silica dust, macrophages form a giant cell capsule around it to limit its movement and damaging effects, protecting the lungs.

Foreign particles entering through the GI tract can reach the liver via portal circulation. In the liver, specialized macrophages known as Kupffer cells engulf and destroy these foreign bodies, preventing them from entering the general circulation.

If a foreign organism enters body tissues, it can eventually reach the lymphatic system. Lymph nodes act as checkposts, containing numerous macrophages that filter and eliminate foreign invaders from the lymph as it passes through the medullary sinuses before exiting.

If foreign bodies enter the bloodstream, the spleen and bone marrow serve as primary checkposts. The spleen, acting as a blood filter, contains macrophages primarily in its red pulp. These splenic macrophages cleanse the blood by engulfing and destroying foreign invaders as blood flows through the splenic red pulp, before the cleansed blood exits.

The monocyte-macrophage system, or reticuloendothelial system, is a comprehensive defense mechanism involving monocytes and specialized endothelial cells. Macrophages are strategically positioned in the skin (histiocytes), lungs (lung macrophages), GI tract/liver (Kupffer cells), lymph nodes, and spleen to protect the body from infections by eliminating foreign invaders.