IRF8 is a transcription factor that functions as both an activator and repressor of immune gene expression, primarily regulating type I interferon responses and dendritic cell differentiation. IRF8 binds to interferon consensus sequences (ICS) in regulatory regions of type I interferon and MHC class I genes 1. It plays a critical role in CD8+ dendritic cell differentiation by cooperating with BATF-JUNB heterodimers to recognize AICE regulatory elements 2. IRF8 is essential for plasmacytoid dendritic cell (pDC) development, which are primary type I interferon producers during viral infection 3. Additionally, IRF8 positively regulates macroautophagy in dendritic cells 4 and transcriptionally represses osteoclast differentiation factors including NFATC1 5. Clinically, IRF8 expression distinguishes functionally distinct immune populations. CD103+ dendritic cells depend on IRF8 for development and mediate effective anti-tumor CTL responses 6, while monocyte-derived tumor-associated macrophages expressing IRF8 paradoxically promote CTL exhaustion 7. In the brain, IRF8 establishes the postnatal microglia epigenetic landscape and transcriptional identity through chr16 accessibility changes 8. Mutations in IRF8 cause Immunodeficiency 32A and 32B, highlighting its essential role in immune development. IRF8-based signatures correlate with immunotherapy responsiveness and clinical outcomes across multiple cancer types.