In the summer of 1907, a German doctor named Clemens von Pirquet noticed something strange with one of his patients. The five-year-old boy had previously tested positive for tuberculosis. The test involved injecting a tiny bit of TB protein just under the skin. His antibodies recognized it, activating immune cells which formed a little bump at the injection site. This happens to anyone who has ever been infected with TB. But when Pirquet performed the same test on the boy a second time, no bump.

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