Current status of xenotransplantation from an immunobiological standpoint.

Xenotransplantation, which involves the transplantation of organs, tissues, or cells from animals into humans, is emerging as a promising solution to mitigate the global shortage of human donor organs. Nonetheless, its clinical application is constrained by significant immunological and physiological barriers-most notably, the rapid rejection of xenografts by both the innate and adaptive immune systems. This review discusses critical immunological challenges such as natural antibodies, complement activation, coagulation dysregulation, and cellular immune responses mediated by T cells, macrophages, and natural killer cells. Advances in genetic engineering-such as the development of multi-knockout pigs that lack major xenoantigens and the transgenic expression of human regulatory proteins-have considerably prolonged xenograft survival in preclinical models. Moreover, approaches such as mixed hematopoietic chimerism, thymic transplantation, cell-based therapies, and synthetic particle-based delivery hold promise for inducing immune tolerance. Innovations in pharmacological immunosuppression and bioengineering, including islet encapsulation and thrombo-regulatory modifications, further enhance graft viability. While recent breakthroughs, including pig-to-human transplants, demonstrate considerable clinical potential, challenges remain in achieving long-term graft survival. This review summarizes current progress in xenotransplantation research and outlines future directions to overcome cross-species immune barriers, bringing the field closer to widespread clinical application.