So when transplanting tissue that contains a high concentration of white blood cells such as T cells, there is a high probability that the graft T cells will recognize the host cells of that individual as being foreign, as being pathogenic. And this will begin a defense response in which these T cells will begin attacking and destroying our host cells. And this is of particular importance when transplanting things like the bone marrow because bone marrow actually contains a very high concentration of T cells. So these are the two major problems, two major complications that accompany grafting an organ transplant. Now, another complication that is really no longer a complication because we have ways of dealing with it is infections. But this was the problem when organ transplant and grafting was at its beginning.