The Body's Defense System
Certain cells located strategically throughout the body tissues do police duty. They are on the lookout, so it seems, for germs that may have slipped by the external lines of defense. Some of these cells circulate with the blood. Others are located in the lymph nodes, in the liver, in the spleen, and elsewhere. These members of the body's police force are designated as phagocytes, macrophages, and reticular cells.
Many of these cells actually engulf germs. Often the police cell gives up its own life in taking a germ into custody. In so doing, however, it keeps the germ from doing further damage.
At other times the body's police cells try to deprive germs of access to other parts of the body. For instance, when a pimple or a boil develops, the police cells come into the involved area from distant parts of the body and lay down a barrier. For this reason it is unwise to squeeze a pimple or boil. Such handling tends to break down the barrier and permits the germs to diffuse through surrounding tissues.