Immunology is the study of how the body responds to foreign substances and fights off infection and other disease. Immunologists study the molecules, cells, and organs of the human body that participate in this response.

The beginnings of our understanding of immunity date to 1798, when the English physician Edward Jenner (1749-1823) published a report that people could be protected from deadly smallpox by sticking them with a needle dipped in the material from a cowpox boil. The French biologist and chemist Louis Pasteur (1822-1895) theorized that such immunization protects people against disease by exposing them to a version of a microbe that is harmless but is enough like the disease-causing organism, or pathogen, that the immune system learns to fight it. Modern vaccines against diseases such as measles, polio, and chickenpox are based on this principle.

In the late nineteenth century, a scientific debate was waged between the...