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 also stimulate Lymphocytes and other immune cells that react to pathogens as what is called an antigen-presenting cell, also known as a histiocyte. Macrophages are located throughout the bloodstream, and may be fixed or they can be freely motile and capable of amoeboid-like movement. Definition of Macrophage and Its Role in the Body The best definition of macrophage is a white blood cell operating within the body tissues. 2  In humans, macrophages measure approximately 21 micrometers in diameter.