Updating the Discontinuity Theory to the Extended Immunity: The Symmunobiome Concept

The immune system (IS) is commonly understood as a system composed of specific cells and tissues that have evolved to contrast pathogens and defend the host. By virtue of this capacity, it has come to be considered capable of making an essential distinction, that between self versus non-self, which would contribute to a clear identity of the organism. However, in the wake of evolution and ecology, growing evidence suggests that the so-called immune system, which also evolved from symbiotic interactions with external agents, is not just a defensive system that merely protects the organism but, on the contrary, is involved in many global regulatory and homeostatic functions. Moreover, in performing these many functions, IS is not only an ensemble of host cells and tissues but functionally is constitutively determined by the interaction with a set of associated microorganisms, that is, the human microbiome. In this scenario, it is open-and-shut that the microbiome itself is a functional part of this extended immune system. Organisms and microbiomes together, therefore, form a functional whole, which constitutes a privileged form of biological organization. In light of this evidence showing the inadequacy of traditional accounts, we propose to extend and supplement the current IS conceptualization by introducing the notion of the symmunobiome. With this term, we intend to characterize the microbiome's own and unavoidable component to overall immune functionality. Therefore, we suggest a new immune system determination, articulated in three linked pillars—adaptive immunity, innate immunity, and symmunobiome—to better grasp the diverse functionality of extended immunity.