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Differences in MHC class I molecules between donor and recipient cells can lead to organ or tissue rejection due to the immune response triggered by the recipient's immune system. MHC class I molecules are present on the surface of almost all nucleated cells and aid the immune system in distinguishing between self and non-self cells. In transplantation, the recipient's immune system recognizes foreign MHC class I molecules on donor cells as non-self, activating cytotoxic T lymphocytes (CTLs). These CTLs then release toxic substances to kill the donor cells within the transplanted organ or tissue, resulting in rejection. Therefore, matching MHC class I molecules closely between donor and recipient is crucial to minimize the risk of organ rejection.