 Cells, organs, and perhaps bodily systems, such as the respiratory system or the digestive system, are physiological individuals that have most if not all of these properties that characterize living agents. As physiological individuals, organisms also share these properties, but are distinguished by one or more further characteristics, such as possessing an immune system (as Pradeu emphasizes) or having a life cycle, one that is typically demarcated through reproduction, which is the focus of  section 6.2  below.