IFITM3(Interferon induced transmembrane protein 3) is an interferon-stimulated restriction factor that localizes mainly to endosomal and lysosomal membranes, where it modulates membrane properties to limit viral entry and fusion. It functions downstream of type I and type II interferon signaling, linking innate immune sensing to cellular antiviral defenses and influencing endocytic trafficking and membrane remodeling. IFITM3 activity has been studied extensively in the context of respiratory virus susceptibility and severity, including influenza virus and coronaviruses, and it can shape inflammatory responses in infected tissues. Beyond infection biology, altered IFITM3 expression has been associated with tumor cell invasion, immune microenvironment interactions, and stress-adaptive programs, making it relevant for studies of immuno-oncology and cell-intrinsic defense pathways.