将一个物种的组织移植到另一个物种体内，例如，从猪到人。和同种移植不同的是，异种之间存在很大差异，从而使得这种移植成功的可能性很小。负责组织排斥的免役系统将很容易地识别出外来组织并强烈排斥它。既然动物可以为移植提供无尽的来源，异种移植一直是人们梦寐以求的事。猪虽然看上去和人有着很大的差异，却有着相似的器官结构，因而成为该领域内研究的焦点；猴子是另一类有吸引力的种群。

A tissue transplant from one species to another, e.g. from pig to human. Because of the greater differences between species, as opposed to within a species, these transplants have the least chance of working. The immune system, which is responsible for tissue rejection, will easily recognize the tissue as foreign and will reject it vigorously. Thus xenograft, or xenotransplantation is a sort of holy grail for transplantation, since animals would provide an endless supply of organs for transplantation. Pigs, although seemingly very different from humans, have similar organ organization and so remain a focus for research in this area; monkeys are another attractive group.