Immune tolerance was thought to prevent self-directed immune reactions from becoming a major cause of disease.

Early immunologists were learning that the blood could contain antibodies: immune proteins that recognize and bind particular targets, such as molecules from microbes or their toxins. Paul Ehrlich argued that the immune system must have powerful safeguards preventing those same weapons from being directed against the body’s own tissues. He called this principle horror autotoxicus—roughly, a “fear of self-poisoning.”

Ehrlich’s actual position was more nuanced than the later slogan that autoimmunity was simply impossible, but the idea strongly shaped early thinking about self-directed immune reactions.

Later research showed that those safeguards can fail. Scientists found antibodies that reacted with a person’s own cells and tissues, and experiments demonstrated that self-directed immune responses could produce organ-specific disease.

The safeguards themselves are real. The immune system normally limits or removes many cells that react strongly against the body. Autoimmune disease occurs when that control breaks down.