The knowledge of acquired resistance to disease and the practices connected with it are very old. Ancient medical systems of both East and West offered explanations of why no one got smallpox twice and how one might be protected from getting it at all. But the history of immunology as a science began only 100 years ago with the experiments of Louis Pasteur.

This history falls into two distinct periods, roughly before and after World War II. It begins with a fanfare, with the production of protective vaccines and antisera, probably the earliest example of truly effective medical treatment. The great theoreticians of the period were biochemists, investigating the chemistry of antigen and antibody and the nature of the antigen–antibody reaction. The part played by blood cells in the body’s defenses had been in the foreground of discussions before the turn of the century, but it was pushed into the shade by the discovery of the striking therapeutic effects of immune sera.

After World War II, when it became clear that antibiotics could control infections for which there was no serological treatment, interest in immunological methods of treatment waned and focused instead on the cellular biology of the immune process as a part of general biology. A single theory came to unite all of immunology, making possible the striking expansion of the field that took place in the 1970s.