The immune system is the body's biological defense mechanism that protects against foreign invaders. Only in the last century have the components of that system and the ways in which they work been discovered, and more remains to be clarified.

Since ancient times medical observers had noticed that the body seemed to have powers to protect itself and resist disease. In particular, people who survived some infectious diseases did not suffer from those diseases again during their lifetime. This led to the practice of variolation in Asia, whereby people were injected with a mild case of smallpox to prevent the later development of a severe case of the disease. Lady Mary Wortley-Montague introduced variolation to Britain from the Ottoman Empire in 1720. The procedure was rather dangerous, however, because the injected person could develop an acute rather than mild case of smallpox, which could lead to an...