Defense
Against Pathogens:
The Human Immune System Cellular and
Molecular Basis of Immunity
Living
organisms are subject to infections caused by viruses, bacteria,
protists, fungi, and animal parasites. Immunity, which
results from the combined activities of many different cells,
has been evolved in vertebrates to recognize and destroy these
infectious agents (called as pathogens). Some of these cells circulate
in body fluids and patrol the body, whereas others reside at specialized
tissues and organs. These dispersed cells in together with the
discrete organs form the immune system.
The outer surface of the body and the linings of the internal
tracts, in fact, have already provided an excellent barrier to
exclude the invasions of pathogens. However, when these surface
barriers are breached, a series of immune responses will be initiated
to combat against these invaded infectious agents. The weapons
of the immune system include cells that kill or ingest infected
cells, and soluble proteins that can neutralize, immobilize, agglutinate,
or kill the pathogens. The cells of the immune system screen and
recognize "foreign" macromolecules, which are different
in structures from those of the body's normal molecules.
The
immune system requires complex and highly specific interactions
between many different immune cells and molecules. A number of
events have to take place before an immune response can be initiated.
Because of this complexity, processes of immune response are particularly
vulnerable to disruptions at different stages by many different
factors. The more obvious of these immune dysfunctions is autoimmune
diseases, which result when the body mounts an immune response
against part of itself. Some common examples of human autoimmune
diseases include multiple sclerosis (MS), insulin-dependent diabetes
mellitus (IDDM), rheumatoid arthritis, and systemic lupus erythematosus
(SLE).
