When considered in the broadest sense as the process whereby organisms maintain functional and organizational integrity against foreign encroachment, immunity is clearly a prerequisite to life itself. Given this, it is not surprising that even the simplest of organisms demonstrate immune functioning, which is consistent with the notion that immunity arose early in the evolution of life on earth (Maier and Watkins 1998). In vertebrates, the immune system has evolved a wide array of separate, but cooperative, mechanisms that serve to attack invading pathogens, destroy tumor cells, and remove and repair damaged tissue. From an evolutionary perspective, the high rate of autoimmune disorders, such as multiple sclerosis and rheumatoid arthritis, suggests that the adaptive advantages of robust immune system functioning outweigh the frequently disastrous consequences engendered when the immune system overreacts and begins attacking the self (Nesse and Williams 1994). The immune system review that follows is necessarily simplified. More extensive discussions of the immune system can be found elsewhere (Abbas and Lichtman 2004; Rabin 1999; Roitt et al. 1998).
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