Publisher Summary The chapter aims to provide the history of theories of antibody formation that calls for attention the many contributions to its progress. The discovery of circulating antibody provides a new and almost impregnable rallying point for those who argued that humoral factors rather than cellular mechanisms are all important in explaining natural and acquired immunity. The theory of antibody formation is universally credited to Breinl and Haurowitz, Mudd, and Alexander, and was rapidly and widely accepted in the early 1930s. In addition, Paul Ehrlich's theory of antibody formation was based upon a Darwinian evolution of the process of intracellular digestion. He pointed out that many different types of nutrients were utilized, apparently specifically, in the metabolism of the cell, and suggested that these could interact and be absorbed by the cell only if structurally-specific receptors exist on the cell surface with which the nutrient molecules can react chemically.