In 1951, a 14-year-old Australian boy named James Harrison awoke from a major chest operation. Doctors had removed one of his lungs in a procedure that had taken several hours — and would keep him hospitalized for three months.

But Harrison was alive, thanks in large part to a vast quantity of transfused blood he had received, his father explained.

“He said that I had 13 units of blood, and my life had been saved by unknown people,” Harrison told CNN’s Sanjay Gupta decades later.

At the time, Australia’s laws required blood donors to be at least 18 years old. It would be four years before Harrison was eligible, but he vowed then that he, too, would become a blood donor when he was old enough.

After turning 18, Harrison made good on his word, donating whole blood regularly with the Australian Red Cross Blood Service. He disliked needles, so he averted his eyes and tried to ignore the pain whenever one was inserted into his arm.

Meanwhile, doctors in Australia were struggling to figure out why thousands of births in the country were resulting in miscarriages, stillbirths or brain defects for the babies.

“In Australia, up until about 1967, there were literally thousands of babies dying each year, doctors didn’t know why, and it was awful,” Jemma Falkenmire, of the Australian Red Cross Blood Service, told Gupta. “Women were having numerous miscarriages, and babies were being born with brain damage.”

The babies, it turned out, were suffering from Haemolytic Disease of the Newborn, or HDN. The condition most often arises when a woman with an Rh negative blood type becomes pregnant with a baby who has Rh positive blood, and the incompatibility causes the mother’s body to reject the fetus’s red blood cells.

Doctors realized, however, that it might be possible to prevent HDN by injecting the pregnant woman with a treatment made from donated plasma with a rare antibody.

Researchers scoured blood banks to see whose blood might contain this antibody — and found a donor in New South Wales by the name of James Harrison.

By then, Harrison had been donating whole blood regularly for more than a decade. He has said he didn’t think twice when scientists reached out to him to ask if he would participate in what would become known as the Anti-D program.

“They asked me to be a guinea pig, and I’ve been donating ever since,” Harrison said.

Before long, researchers had developed an injection called Anti-D using plasma from Harrison’s donated blood. The first dose was given to a pregnant woman at Royal Prince Alfred Hospital in 1967, according to Robyn Barlow, the Rh program coordinator who found Harrison.

Harrison continued donating for more than 60 years and his plasma has been used to make millions of Anti-D injections, according to the Red Cross. Because about 17 percent of pregnant women in Australia require the anti-D injections, the blood service estimates Harrison has helped 2.4 million babies in the country.

“Every ampul of Anti-D ever made in Australia has James in it,” Barlow said.. “He has saved millions of babies. I cry just thinking about it.”