The birth of immunology: Metchnikoff, the embryologist

Metchnikoff must be viewed first as an embryologist, who, influenced by the Darwinian currents of the 1860s and 1870s, sought to establish a genetic and embryologic unity in phylogeny. His principal early theory that the mesoderm was the origin of endodermal structures enabled him to extend the observation of mesodermal digestive processes to a theory of immunity. Observation of amoeboid phagocytosis was not novel, but Metchnikoff's scientific investigations had prepared him to interpret this activity as a manifestation of a generalized property of the mesoderm. Earlier observers noted the presence of microorganisms and particles in leukocytes, and the notion of phagocytosis had previously been entertained, but only Metchnikoff recognized the importance of phagocytosis in a general scheme of inflammation and to develop an experimental model for its investigation. The observation was thus viewed not solely as an issue of pathology, but rather as a contribution to Metchnikoff's general idea of genetic unity and his hypothesis of a primordial multicellular organism, Parenchymella, later called Phagocytella. It is striking that he ultimately viewed phagocytosis as a question of immunity, considering the context of his research activities, which had been confined to evolution and biology of development. To demonstrate how the famous Messina experiments were extended to a new theory of immunity requires formulating Metchnikoff's recognition of both the importance of phagocytosis for his mesodermal theory and a more general theory of pathology. The result was the genesis of a new idea, immunity.