Why did the cowpox antibodies prevent people from getting smallpox as well?

Cowpox antibodies provided cross-protection against smallpox because both cowpox and smallpox are caused by viruses that belong to the same family, Poxviridae. The viruses share similar antigens, particularly on their outer surface proteins. When someone is infected with cowpox, their immune system produces antibodies that recognize and bind to these shared antigens.

When a person vaccinated against cowpox is later exposed to the smallpox virus, their immune system can quickly recognize the similar antigens on the smallpox virus. The antibodies produced against cowpox can bind to the smallpox virus, neutralizing its infectivity and preventing it from establishing an infection. This cross-protection is the basis for the effectiveness of the cowpox vaccine in preventing smallpox.

Edward Jenner, an English physician, observed that milkmaids who had been exposed to cowpox, a mild disease that causes sores on the hands, seemed to be immune to smallpox. He hypothesized that exposure to cowpox protected against smallpox and conducted an experiment in 1796 where he deliberately infected a young boy with cowpox and then later exposed him to smallpox. The boy did not develop smallpox, demonstrating that cowpox vaccination could provide immunity to smallpox.

This discovery led to the development of the smallpox vaccine, which was widely used and successful in eradicating smallpox from the world. The eradication of smallpox is considered one of the greatest achievements in public health history.