The transplant heart was surgically removed from the donor pig before the surgery on the human patient; pig organs are considered suitable for transplant to humans because they are about the same size and shape.

David Bennett, 57, had the operation in Baltimore, Maryland, on 7 January using a heart that had been genetically modified to boost the chances of acceptance in a human body. The donated heart came from a pig developed by US firm Revivicor. In total, the animal had 10 genes modified. Four of those were inactivated, including one that causes an aggressive immune response and one that would otherwise cause the pig’s heart to continue growing after transplant into a human body.

The surgery took place on Friday (Jan. 7), and after four days the human patient is breathing on his own, although he is still connected to a heart-lung machine to strengthen his blood circulation, according to a statement from the University of Maryland Medical Center (UMMC). The next days and weeks will be critical to whether he survives the operation.

Bennett was approved to have the risky procedure as he was too sick to go on the waiting list to get a human heart. If he continues to stay well, it could open the door to such transplants for a growing pool of other people. It could also lead to pig-to-human transplants of kidneys, livers and lungs in future.

According to livescience