ABSTRACT
The human immune system is designed to protect the body against foreign substances, bacteria and viruses as well as any abnormal cells that might arise within the body itself. The immune component is only one portion of an overall host defense system that includes physical and chemical barriers to infection as well as biologic and cellular response mechanisms. Some of these “layers” of host cell defense are very general and designed to stop any and all foreign organisms, whereas others are highly specific for certain organisms. The first line of defense against invading microorganisms involves the physical barriers presented by the skin on the body surfaces and the mucous membranes that protect openings into the body. These mucous membranes are covered by secretions such as sticky mucus that can trap invading organisms. Digestive enzymes such as
lysozymes
are often found within mucous membrane secretions and provide a chemical barrier to infections. Organisms that are able to penetrate this first line of defense must now face circulating
phagocytic
white blood cells, antimicrobial substances and
natural killer cells
, all of which attack invading organisms in a nonspecific manner. In addition, the entry of these foreign organisms into the body will trigger a significant inflammatory response and possibly fever, both of which are also protective. In addition to the above nonspecific defense mechanisms, specialized cells called
lymphocytes
and antimicrobial proteins called
antibodies
can attack foreign organisms in a highly specific manner.
