May 1796 was not a complete theory of immunity. It was one risky test.
Jenner used material from a cowpox sore on milkmaid Sarah Nelmes to inoculate 8-year-old James Phipps.
After Phipps recovered, Jenner later exposed him to smallpox material in a deliberate challenge.
Phipps did not develop smallpox, which gave support to the milkmaid observation that cowpox could protect against smallpox.
By modern research standards, the test was limited because it was a one-child challenge, not a controlled trial.
Its historical value is narrower than the legend. Jenner tested a protective link; he did not explain the immune system.