What is the most important electrolyte for the maintenance of osmotic pressure and volume in the extracellular fluid compartment? The most abundant positive ion or cation in the extracellular fluid  is sodium (Na+). Chloride (Cl-) is the most abundant negative ion  or anion.
Medical Definition of MACROPHAGE. : a phagocytic tissue cell of the immune system that may be fixed or freely motile, is derived from a monocyte, functions in the destruction of foreign antigens (as bacteria and viruses), and serves as an antigen-presenting cellâsee histiocyte, mononuclear phagocyte system. a phagocytic tissue cell of the immune system that may be fixed or freely motile, is derived from a monocyte, functions in the destruction of foreign antigens (as bacteria and viruses), and serves as an antigen-presenting cell â compare histiocyte. â macÂ·roÂ·phagÂ·ic \Ëma-krÉ-Ëfa-jik\ adjective.
Macrophages are a type of white blood cells that engulf foreign material. They are key players in the immune response to foreign invaders, such as infectious microorganisms Macrophages also release substances that stimulate other cells of the immune system.This information is based on source information from the National Institute of Neurological Disorders and Stroke.hey are key players in the immune response to foreign invaders, such as infectious microorganisms Macrophages also release substances that stimulate other cells of the immune system. This information is based on source information from the National Institute of Neurological Disorders and Stroke.