So our wide blood cells use something called the major histocompatibility complex and self antigens to basically distinguish our whole cells from infected cells or pathogens. Now, the problem with grafting or transplantation is when we actually transfer that tissue or that organ, the tissue or the organ on the other individual might have different major histocompatibility complexes and different self antigens. And if these antigens are not compatible, if there is no match, then what the wide blood cells of that host immune system do is they recognize those other cell antigens of the other individual as pathogenic, as being foreign. And they mount a defensive response. They label those cells for destruction and begin destroying that allograph altogether. So, once again, when a tissue or organ is transplanted it has a very high chance of being rejected by the host individual.