And this particular organ contains a high concentration of T cells, of white blood cells. And one example of such an organ, such a tissue is bone marrow. So let's suppose we're transformed, we're transferring bone marrow from one individual to a different individual. The problem with this is the bone marrow contains a high concentration of T cells. And these T cells, when they're transferred into the host individual, the T cells contain receptors that might not recognize the cell's antigens and the major histocompatibility complexes found on the host cells. And what that means is, when the grass cell binds onto the host cell, it will begin to release these enzymes that will destroy the host cells of that host individual.