From Darwin and Metchnikoff to Burnet and beyond.

Phagocytosis in unicellular animals represents the most ancient and ubiquitous form of defense against foreign material. Unicellular invertebrates can phagocytose for food and defense. Multicellular invertebrates and vertebrates possess phagocytic cells and have evolved more complex functions attributed to immunodefense cells that specialized into cellular and humoral immune responses. Thus all animals possess: innate, natural, nonspecific (no memory) nonanticipatory, nonclonal, germline (hard wired) host defense functions. In addition, all vertebrates possess: adaptive, induced, specific (memory), anticipatory, clonal, somatic (flexible) immune responses. A similar situation exists with respect to components of the signaling system, immunity and development. With multicellularity, clearly numerous immune response characteristics are not possible in unicellular forms or even those that straddle the divide between unicellularity and multicellularity, beginning with colonial/social protozoans. Still, it is instructive to elucidate a hierarchy of animals based upon immunologic characteristics and how they parallel other physiological traits. Evidence is presented that the most primitive of invertebrates prior to the evolution of multicellular organisms possess varying degrees of complexity at the molecular level of those hallmarks that now characterize the immune system.