Recent studies have revealed that the immune system plays a critical role in various physiological processes beyond its classical pathogen control activity. Even under a sterile condition, various cells and tissues can utilize the immune system to meet specific demand for proper physiological functions. Particularly, a strong link between immunity and metabolism has been identified. Studies have found reciprocal regulation between these two systems For example, immune signals can regulate metabolism, and metabolism (cellular or systemic) can regulate immunity. In this review, we will summarize recent findings on this reciprocal regulation between immunity and metabolism and discuss potential biological rules behind this interaction with integrative perspectives.