A genetically engineered pig kidney appears healthy during a transplant operation at NYU Langone in New York, U.S. For the first time, a pig kidney has been transplanted into a human without triggering immediate rejection by the recipient's immune system, a potentially major advance that could eventually help alleviate a dire shortage of human organs for transplant. Joe Carrotta for NYU Langone Health/Handout via REUTERS

The surgical team examines the pig kidney for any signs of hyperacute rejection, as the organ was implanted outside the body to allow for observation and tissue sampling during the 54-hour study period, at NYU Langone in New York, U.S. The recipient was a brain-dead patient with signs of kidney dysfunction whose family consented to the experiment before she was due to be taken off of life support. Joe Carrotta for NYU Langone Health/Handout via REUTERS

A genetically engineered pig kidney appears healthy during a transplant operation at NYU Langone in New York, U.S. Test results of the transplanted kidney's function "looked pretty normal," said transplant surgeon Dr. Robert Montgomery, who led the study. Joe Carrotta for NYU Langone Health/Handout via REUTERS

Dr. Robert Montgomery, the Leon H. Pachter, MD, Professor and chair of the Department of Surgery at NYU Langone and director of its Transplant Institute, performs the first xenotransplantation of a genetically engineered nonhuman organ to a human at NYU Langone in New York, U.S. Researchers have been working for decades on the possibility of using animal organs for transplants, but have been stymied over how to prevent immediate rejection by the human body. Joe Carrotta for NYU Langone Health/Handout via REUTERS

Dr. Robert Montgomery, the Leon H. Pachter, MD, Professor and chair of the Department of Surgery at NYU Langone and director of its Transplant Institute, prepares sutures for use in the xenotransplantation surgery at NYU Langone in New York, U.S.Joe Carrotta for NYU Langone Health/Handout via REUTERS

A genetically engineered pig kidney is cleaned and prepared for transplantation to a human at NYU Langone in New York, U.S. Joe Carrotta for NYU Langone Health/Handout via REUTERS