It was to account for the protective immunity conferred by Pasteurian vaccines that a new science developed in the 20th century, immunology, and that immunologists conceived the immune system as a defense system, based on the discrimination between self and non-self. This system, indeed protects against infectious diseases and probably against cancers. Sometimes, however, it has deleterious effects such as in aged Covid-19 patients; sometimes, it aims at the wrong target and destroys our own cells; sometimes, it reacts to harmless environmental substances and causes allergies. Conversely, the maternal immune system lets grow for 9 months half-foreign fetuses instead of rejecting them as it rejects other incompatible grafts, and we learnt that we are meta-organisms made of many more microorganisms than we have cells, which control the immune system more than the latter controls them. How can a defense system cause immunopathology, autoimmunity and allergy? How does it manage to spare not only an incompatible fetus, but also the multitude of commensals that live inside us? It was with the aim of answering these questions that this book was written. I propose to view the immune system as a system of relation that deals with living beings, whereas the nervous system deals with physicochemical phenomena. Doing so, the immune system not only adapts the organism to other living beings, it also adapts other living beings to the organism, thanks to powerful effectors. A state of immunity that protects the organism against others is replaced by an endless dynamic process, and the notion of defense by that of a constantly renegotiated compromise that enables the organism to live with others. For the meta-organism that we are, immunity becomes more than a protection, it is a condition of existence.