The term "autoimmunity" has been used to categorize a number of different diseases of unknown etiology. The term as applied to many of these diseases would probably be interpreted best as "autoreactivity," as the clinical phenotypes are often characterized by an inflammatory-like accumulation of activated cells of the immune system at sites of obvious damage of normal cells and/or tissue. The reasons as to why an immune system should attack itself are far from clear, although the association with specific infectious diseases in genetically susceptible people remains perhaps our best lead. The...