We will see in this chapter that if we question the place of the immune system among the other major physiological systems of mammals, it appears as a second system of relations, alongside the nervous system. The nervous system perceives the physical world while the immune system perceives the living world. Both systems modify the organism accordingly, but only the immune system can modify what it perceives. Mounting an immune response is therefore both adapting oneself to others and adapting others to oneself. The immune system can perform this double response because it possesses almost infinite cognitive and effector repertoires.