How did Edward Jenner's cowpox experiments lead to smallpox vaccination?

English surgeon Edward Jenner developed the smallpox vaccine by building on the observation that dairymaids who contracted cowpox—a mild disease—appeared immune to the deadly smallpox virus.123

In 1796, Jenner tested this hypothesis by taking material from a cowpox lesion on a dairymaid and inoculating an eight-year-old boy, James Phipps. After the boy recovered from a mild illness, Jenner exposed him to smallpox matter; the boy did not contract the disease, confirming he had developed immunity.143

This success occurred because cowpox and smallpox share common antigens, which trigger the immune system to provide protection against the more dangerous virus. Jenner published his findings in 1798, coining the term vaccination from the Latin word vacca, meaning "cow". His work replaced the riskier practice of variolation and laid the essential foundation for modern vaccines.143