ABSTRACT
The human body is constantly exposed to a wide variety of pathogenic and non-pathogenic microorganisms and material. Pathogenic and non-pathogenic microorganisms and material include bacteria, viruses, fungi, parasites and non-living substances such as toxins, chemicals and drugs. Immunity can be conveniently divided into two complementary systems: non-specific immunity and specific immunity. The most obvious type of non-specific immunity is made up of the physical and chemical barriers of our integumentary system. Specific immunity is also known as acquired or adaptive immunity and is only available at birth in an extremely limited fashion. The key mechanism that enables the immune system to avoid disease and maintain health is its ability to distinguish between the body's own cells (self) and those that represent a potential threat (non-self). Natural killer cells are large granular lymphocytes that mature in the bone marrow, lymph nodes, spleen, tonsils and thymus gland.
