The immune system is a network of molecules, cells, and organs that weaves through the human body to detect and eliminate deviations from the steady-state and in harmless interactions of humans with their commensals and nutrients. This introductory chapter broadly summarizes which metabolic pathways constitute the roots of immune functions, namely pathogen sensing, interactions between immune cells, and effector functions to eliminate pathogenic threats. Furthermore, this chapter provides insights into how the immune system achieves maximal flexibility in responding to tissue changes while maintaining self-tolerance. Depending on the nature of human diseases, such as lack of immune control or autoimmunity, the border between immune reactivity and tolerance is therapeutically moved and more treatments that harness the immune system should be developed in the future.