In this paper, we discuss the features of the immune system; its system aspect (compatible with Jerne's network view), its process aspect, and its design aspect (compatible with Metchinikoff's self-defining view). Our applications of these three aspects are also presented briefly. Since these features of the immune system agree with the concept of autonomous decentralized systems, we suggest that the immune system can be a typical model for autonomous decentralized systems.In the self-defining process, agents can refer to the self-information, and can interact with the environment. We propose that this self-defining process extracted from the immune system can be a candidate for design paradigm for autonomous decentralized systems where full specification of the total system is not only available but also inadequate. We also suggest that the internet would be a typical example designed by this paradigm.