The history of immunology 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. In the nineteenth century, the startling success of the serum treatment of diphtheria had given rise not only to the practical problems of standardization and their solution, and to the international organization to coordinate the work, but also to a theoretical interest in the antigen-antibody reaction and the nature of specificity. The clonal selection theory had the effect of enormously enlarging the field, uniting its domains, and linking immunology to the broader biological sciences. One of the most significant continuities has been in blood grouping. As well as being directly utilized in hospital blood banking, the thinking about and techniques of blood group serology laid the conceptual foundation for human genetics in general.