The hero of the following account, Homo immunologicus, who must give his life, with all its dangers and surfeits, a symbolic framework, is the human being that struggles with itself in concern for its form. We will characterize it more closely as the ethical human being, or rather Homo repetitious, Homo artista, the human in training. None of the circulating theories of behaviour or action is capable of grasping the practising human - on the contrary: we will understand why previous theories had to make it vanish systematically, regardless of whether they divided the field of observation into work and interaction, processes and communications, or active and contemplative life. With a concept of practice based on a broad anthropological foundation, we finally have the right instrument to overcome the gap, supposedly unbridgeable by methodological means, between biological and cultural phenomena of immunity - that is, between natural processes on the one hand and actions on the other.