Since marrow stem cells spawn every kind of blood cell, they can, when transplanted, restore life to a dying host. Blood is constantly being renewed by stem cells in our bone marrow: red cells turn over every few months, platelets and most white cells every few days. Platelets and proteins in plasma form clots that can prevent fatal hemorrhages. White cells defend us against invasion by lethal pathogens. Red cells carry oxygen, required for our heart to beat and our brain to function.
During my training as a hematologist at U.C.L.A., forty years ago, a senior faculty member introduced the program of study by citing a verse from Leviticus: “The life of the flesh is in the blood.” For the assembled young physicians, this was a biological truth.
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