The objective of this chapter is to start by taking stock. A good century of research has produced a considerable amount of experimental data, established facts, elucidated mechanisms and supported conclusions. I will therefore forget that even when it protects the system is immune. I will try to rebuild a biological system in the opposite way, i.e., starting from proven facts rather than from expected effects. A system that would also confer a protective immunity, of course. I will therefore first gather the bases of a physiological system without metaphysical or ideological a priori, a system to which no function would be assigned to justify its existence, a biological system freed from end causes.