Horses were injected with gradually increasing doses of virulent bacteria. The bacteria made the toxin. In turn, the horse’s immune system generated antibodies that bound to and neutralized the toxin. The horse was then bled, solids removed from the blood until only the serum remained, and this was then purified into the antitoxin that had become so common and lifesaving. An identical process produced tetanus antitoxin, Flexner’s serum against meningitis, and several other sera or antitoxins. Scientists were vaccinating the horse against a disease, then extracting the horse antibodies and
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