Self-attacking Immune Cells Turned Off in Healthy People; Turned On in Rheumatoid Arthritis

Rheumatoid arthritis and lupus are autoimmune diseases -- conditions associated with self-attacking immune cells. According to a report in the December 22, 2008 edition of Journal of Experimental Medicine, healthy people also have the cells but they are kept in an "off state". Why are the cells kept off in some people and turned on in others?