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The '''immune system''' is the collection of organs and tissues involved in the adaptive defense of a body against foreign biological material. It may be broken down into the adaptive immune system (only found in vertebrates[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9297968]), composed of four lymphoid organs ([[thymus]], [[lymph node]]s, [[spleen]] and submucosal lymphoid nodules) and the group motile cells that are involved in the body's defense against foreign bodies. The term may also be used to refer to the totality of a body's defense systems, encompassing both the adaptive immune system and other passive defenses, such as the skin.
In multicellular [[organism]]s, the immune system is an organ system that acts as a defense against foreign [[pathogen]]s (such as [[virus (biology)|viruses]], [[bacterium|bacteria]], [[parasite]]s), some [[poison]]s, as well as [[cancer]]. Components of the immune system also function in the return of extracellular fluid to the blood.
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