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 The macrophage is one of the most important cells of the immune system. For many years it has been known as the main phagocyte of the immune system. Macrophages eat and destroy pathogens, such as bacteria, viruses and fungi, thus they are a critical part of the body's defense against microorganisms.acrophages are produced in the bone marrow, where red blood cells, platelets and granulocytes are made. In the bone marrow, macrophage precursor cells are called promonocytes. Promonocytes eventually mature and migrate into the blood, where they are called monocytes. Monocytes are only found in the blood.