Normal human cells can also become foreign invaders. When an organ such as a liver is transplant (moved) form one person to another, cells from the transplanted organ are seen as enemy invaders by the immune system of the person receiving the organ. The cells of the transplanted organ set off an immunological attack. Killer T cells and antibodies rush to the site, causing organ rejection, and the organ dies. Physicians solve this problem in part with medicines that suppress, or slow down, the immune system.