A century ago the visionary bacteriologist Paul Ehrlich aptly coined that term to describe an immune system attack against a person's own tissues. Ehrlich thought such autoimmunity--another term he coined--was biologically possible yet was somehow kept in check, but the medical community misconstrued his two-sided idea, believing instead that autoimmunity had to be inherently impossible. After all, what wrong turn of evolution would permit even the chance of horrendous, built-in self-destruction?

Slowly, though, a number of mysterious ailments came to be recognized as examples of horror autotoxicus--among them multiple sclerosis, insulin-dependent diabetes (the form that commonly strikes in youth) and rheumatoid arthritis. Investigators learned, too, that these diseases usually stem from the renegade actions of white blood cells known as CD4+ T lymphocytes (so named because they display a molecule called CD4 and mature in the thymus). Normal versions of these cells serve as officers in the immune system's armed forces, responsible for unleashing the system's combat troops against disease-causing microorganisms. But sometimes the cells turn against components of the body.