The present invention relates to a composition for suppressing an immune response, containing a Janus Kinase (JAK) inhibitor, and a composition for protecting the pancreatic islet, containing a JAK inhibitor. The composition of the present invention has an excellent effect of suppressing blood- and cell-mediated immune responses, including instant blood mediated inflammatory reaction (IBMIR), which is the main cause of pancreatic islet loss, and, unlike traditional immunosuppressants, has effects of maintaining the function of the pancreatic islet and promoting the survival rate of the pancreatic islet, thereby being usable in the suppression of an immune response and the protection of pancreatic islet cells during the xenotransplantation of the pancreatic islet.