Introduction

Xenotransplantation refers to transplantation of cells, tissues, or organs from individuals of one species into individuals of another species. Xenotransplantation has long been envisioned as a way of addressing the severe shortage of human organs and tissues for transplantation, as the number of available organs is as low as 5% of the number needed.

The first serious efforts at xenotransplantation were made in the early 20th century; however, these attempts at xenotransplantation failed, as did early attempts at human-to-human transplantation (allotransplantation), because the immune system of the recipients inevitably rejected the graft. This barrier was addressed in part by the advent of immunosuppressive drugs in the late 1950s, and the era of clinical allotransplantation began. However, as human donors were limited, animals, such as chimpanzees and baboons, were used once again as a source. Those primate-to-human xenografts functioned for weeks to months, but ultimately failed, whereas human-to-human transplants sometimes functioned indefinitely. This experience suggested that the immunological barrier to xenotransplantation was more severe than the barrier to allotransplantation. Recent years have brought a better understanding of the barriers to xenotransplantation and have raised the possibility of applying genetic engineering to that barrier.

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