In a biological sense, what does it mean to be “in good health”? This far-reaching question is the focus of the “Laboratoire d'Excellence” project Milieu Intérieur (“Environment Within”). The Milieu Intérieur cohort is the first French cohort of its kind, comprised of a thousand healthy donors. Studying this cohort will help define the parameters of a healthy immune system. The recent analysis of the initial results generated from the cohort represents an important milestone for the scientists in this consortium.