This chapter embarks on an intellectual exploration of the HEARTY City concept, seeking to establish its theoretical underpinnings through an analogy to the human immune system. While the HEARTY City has already been proposed as a forward-thinking model for integrated urban planning, this discourse aims to deepen our understanding by framing its six core pillars—H for Healthy City, E for Eco-City, A for AI-driven City, R for Resilient City, T for Tourism City, and Y for Yearning City—within the adaptive and self-regulating nature of the immune system. Just as the immune system is a sophisticated network that safeguards, adapts, and evolves in response to both internal and external challenges, so too does the HEARTY City aspire to cultivate urban environments that are capable of thriving amid an ever-changing world. By invoking the dynamics of immunity—defense, adaptation, regeneration, and integration—this chapter seeks to reveal a deeper, holistic understanding of the HEARTY City. It is not merely a collection of static elements, but it is an interconnected, responsive system—one that nurtures the long-term sustainability and vitality of the city, continually balancing the needs of the present with the aspirations of the future. In this light, the HEARTY City becomes more than a model of urban planning; it is a living organism, a dynamic entity in perpetual self-renewal.