Xenotransplantation

Pigs are anatomically close enough to humans that their kidneys and hearts can sustain human circulation, but the immune system rejects foreign tissue violently — hyperacute rejection can destroy an organ in minutes. The fix is genetic: researchers knock out the pig genes that trigger human immune attack and splice in human genes that help the organ pass as self. Each round of editing narrows the rejection window. The hard parts that remain are chronic rejection over months, risk of pig viruses crossing into the human host, and organ sizing.