The human body has the ability to resist almost all types of organisms or toxins that tend to damage the tissues and organs. This capability is called immunity. Much of immunity is acquired immunity thatdoes not develop until after the body is first attacked by a bacterium, virus, or toxin, often requiring weeks or months to develop the immunity. An additional portion of immunity results from general processes, rather than from processes directed at specific disease organisms. This is called innate immunity. It includes the following:
1. Phagocytosis of bacteria and other invaders by white blood cells and cells of the tissue macrophage system.
2. Destruction of swallowed organisms by the acid secretions of the stomach and the digestive enzymes.
3. Resistance of the skin to invasion by organisms.
4. Presence in the blood of certain chemical compounds that attach to foreign organisms or toxins and destroy them. Some of these compounds 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 complementcomplex that is described later, a system of about 20 proteins that can beactivated in various ways to destroy bacteria; and (4) natural killerlymphocytes that can recognize and destroy foreign cells, tumor cells, andeven 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 dis-temperâ€”a viral disease that kills a large percentage of dogs that become afflicted with it. Conversely, many lower 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.