Recent studies have revealed that neuroimmune system is involved in the brain development and the pathogenesis of neurological diseases. However, it remains unclear how neuroimmune system modulates brain functions at a molecular level. We identified the role of immune cells in brain development and inflammatory neurological diseases. We demonstrated that B cells were abundant in the developing brain, and contribute to myelination by promoting the proliferation of oligodendrocyte precursor cells. In other study, we identified the role of microglia, which are immune cells in central nervous system, in the progression of autoimmune encephalomyelitis. We depleted microglia by PLX3397, an inhibitor of colony-stimulating factor receptor 1 (CSF-1R), in autoimmune encephalomyelitis, and showed that microglia regulate the T cell proliferation and differentiation during disease progression. In this article, we introduce the recent findings of the role of neuroimmune system in the brain development and pathogenesis of neurological diseases.