There are enzymes in cells called proteasomes that degrade intracellular proteins that are damaged, not needed anymore or just old and ready to be replaced. The

There are enzymes in cells called proteasomes that degrade intracellular proteins that are damaged, not needed anymore or just old and ready to be replaced. The rest products are peptides, short chains of amino acids, some of which become bound to molecule called the Major Histocompatibility Complex 1, which is then transported to and presented on the surface of the cell. It functions more or less as an ID badge of the cell. Foreign cells, cancerous cells or cells infested by bacteria or viruses will present unknown proteins, which will trigger an immune response. An obvious solution would be to deactivate the MHC 1 in transplanted foreign tissue, but sadly that wouldn't work because the Natural killer cells in our immune system destroy cells without MHC 1. This is probably an adaption that evolved as bacteria evolved the capability to downregulate MHC 1 molecules to evade detection by other parts of the immune system.