THE DIVISIONS OF THE IMMUNE SYSTEM

The mammalian immune system has two overarching divisions. The innate part (left side) acts near entry points into the body and is always at the ready. If it fails to contain a pathogen, the adaptive division (right side) kicks in, mounting a later but highly-targeted attack against the specific invader.

Innate Immune System
This system includes, among other components, antimicrobial molecules and various phagocytes (cells that ingest and destroy pathogens). These cells, such as dendritic cells and macrophages, also activate an inflammatory response, secreting proteins called cytokines that trigger an influx of defensive cells from the blood. Among the recruits are more phagocytes -- notably monocytes (which can mature into macrophages) and neutrophils.

Adaptive Immune System
This system "stars" B cells and T cells. Activated B cells secrete antibody molecules that bind to antigens -- specific components unique to a given intruder -- and destroy the invader directly or mark it for attack by others. T cells help attack infected cells. T and B cells spawn "memory" cells that promptly eliminate invaders encountered before.

Source:  Scientific American 2004

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The immune system is composed of a complex constellation of cells, organs and tissues, arranged in an elaborate and dynamic communications network and equipped to optimize the response against invasion by pathogenic organisms.  The immune system is, in its simplest form, a cascade of detection and adaptation, culminating in a system that is remarkably effective.  The immune response can be manipulated to suppress unwanted responses in autoimmunity, allergy and transplant rejection, and to stimulate protective responses against pathogens that largely elude the immune system.