This intellectual history is the first critical study of the work of Elie Metchnikoff, the founding father of modern immunology. Metchnikoff authored and championed the theory that phagocytic cells actively defend the host body against pathogens and diseased cells. In this scientific biography, Tauber and Chernyak explore Metchnikoff's development as an embryologist, showing how it prepared him to propose his theory of host-pathogen interaction. They discuss the profound impact of Darwin's theory of evolution on his progress, and the influence of 19th-century debates on vitalism, teleology, and mechanism. As a case study of scientific discovery, this work offers insight into the process of creative science and its dependence on cultural and philosophic sources.