His plasma, the liquid part of his blood, carried a rare antibody called anti-D.
It may have formed because of the blood he received as a boy. If so, the strangers who saved his life had given him something more.
To understand why it mattered, you have to understand a cruel problem in pregnancy.
When a mother is Rh-negative and her baby is Rh-positive, her immune system can treat her own baby as a threat, and attack the baby's red blood cells.
It's called rhesus disease. It can cause brain damage, stillbirth, or death soon after birth. And it tends to get worse with each pregnancy.
Anti-D can stop it. Given to the mother, it helps keep her body from launching that attack.
But anti-D couldn't simply be made in a factory. It had to come from people.
People like James.