The human body has a natural ability to resist almost all types of organisms or toxins that invaded the body which may cause damage to the body tissues and organs. This ability is called immunity. Much of immunity is acquired immunity that does not develop until after the body is first attacked by a bacterium, virus, or toxin; often weeks or months are required for the immunity to develop. An additional element of immunity that results from general processes, rather than from processes directed at specific disease organisms, is called innate immunity. It includes the following aspects:

• Phagocytosis of bacteria and other invaders by white blood cells and cells of the tissue macrophage system.
• Destruction of swallowed organisms by the acid secretions of the stomach and the digestive enzymes.
• Resistance of the skin to invasion by organisms.
• Presence in the blood of certain chemicals and cells that attach to foreign organisms or toxins and destroy them. Some of these are
  (1) lysozyme, a mucolytic polysaccharide that attacks bacteria and causes them to dissolute;
  (2) basic polypeptides, which react with and inactivate certain types of gram-positive bacteria;
  (3) the complement complex that will be described later, is a system of about 20 proteins that can be activated in various ways to destroy bacteria; and
  (4) natural killer lymphocytes that can detect and destroy foreign cells, tumor cells, and even some infected cells.

This innate immunity makes the human body resistant to such diseases as some paralytic viral infections of animals, hog cholera, cattle plague, and distemper—a viral disease that kills a large percentage of dogs that become afflicted with it. Conversely, many animals are resistant or even immune to many human diseases, such as poliomyelitis, mumps, human cholera, measles, and syphilis, which are very damaging or even lethal to human beings.