ISG15 is a ubiquitin-like protein that is conjugated to intracellular target proteins upon activation by interferon-alpha and interferon-beta. ISG15 plays a crucial role in immune response by functioning as a ubiquitin-like modifier, conjugating to target proteins such as JAK1 and MAPK3/ERK1 through a unique enzymatic pathway distinct from ubiquitin. This modification regulates various cellular processes, including protein stability and signaling pathways, which influence immune cell activation and response. ISG15 exhibits specific chemotactic activity towards neutrophils, promoting activation and release of eosinophil chemotactic factors, thereby enhancing immune response.