When he was 14, James Harrison needed surgery. And as he would come to find out, he would also need a significant amount of strangers’ blood to survive it.

After he had recovered and as soon as he became an adult, Mr. Harrison felt compelled to pay it forward, he said. For the next 60 years he suppressed his strong distaste for needles — he says he has never watched one go into his arm — and gave blood every few weeks at locations across Australia.

Along the way, medical professionals made a stunning discovery: Mr. Harrison’s blood contained a rare antibody necessary to make a pioneering medication that officials at the Australian Red Cross Blood Service said had helped save more than two million babies from a potentially fatal disease.

They said more than three million doses of Anti-D, as the medication containing Mr. Harrison’s blood is called, have been issued to mothers since 1967.