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In page Autoimmunity:

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In the later 19th century, it was believed that the immune system was unable to react against the body's own tissues. Paul Ehrlich, at the turn of the 20th century, proposed the concept of horror autotoxicus. Ehrlich later adjusted his theory to recognize the possibility of autoimmune tissue attacks, but believed certain innate protection mechanisms would prevent the autoimmune response from becoming pathological.[citation needed]