Without understanding the mechanisms of immunity, Dr. Edward Jenner in the late 18th century turned it to the service of medicine when he vaccinated his patients against smallpox with the similar virus of cowpox. He had acted on the popular belief in his part of Gloucester that people who contracted the much milder cowpox where thenceforth immune to smallpox. In our century artificial immunisation has become the most effective protection against many infectious diseases. It takes two forms: passive immunisation in which the patient is inoculated with serum containing antibodies to a particular disease organism; and active immunisation in which the inoculant used contains live or killed virus, or more often a weakened strain of the virus. The nature of immunity is under increasingly intensive study as medical scientists strive to improve the results of organ transplantation, now limited by the patient's immune reaction to foreign tissue.
