The reticulo-endothelial system is a generic name for those cells in the body that take up dyes and other foreign material from the blood stream and other body fluids; they are also known as macrophage. Blood monocytes are functionally part of the system as are macrophages in the lymph nodes, spleen, bone marrow, liver (Kupffer cells), and lung alveoli. When foreign material (e.g., bacteria) is introduced into the blood stream, macrophages rapidly take it up and destroy it with intracellular enzymes. This constitutes a primary defense system and plays a role in the immune system. Similarly, particulate matter in the lungs or lived is cleared by macrophages.