The Immune system is the body’s main defense against all foreign substances.   The largest organ in the Immune system is the skin.  The skin acts as a protective barrier for the rest of the body, keeping out literally billions of microscopic bacteria cells.  Some of which are disease causing pathogens.  Also involved in the immune system is the bloodstream which carries billions of tiny cells specializing in the removal of pathogens.  These cells include white blood cells, platelets, and antibodies.