The central nervous system (CNS) is an immune-privileged area. The blood-brain barrier (BBB) is thought to separate the CNS from any systemic inflammatory states to maintain homeostasis within this specialized, vulnerable organ. However, accumulating studies have challenged this concept by demonstrating systemic inflammatory effects on brain. Moreover, the coronavirus disease pandemic caused by severe acute respiratory syndrome coronavirus 2 in 2019 has rapidly evoked attention toward the BBB as the systemic-CNS immunological interface. In this review, we focus on microglia, the sole immune cells in the CNS, and briefly introduce our new findings regarding microglial BBB regulation in systemic inflammation. With a close eye on associated literature, we carefully rethink the traditional immune system in the CNS and suggest a new possible mechanism of systemic-CNS immune cell interaction, while an understanding of the BBB develops.