Blood has always had a special mystique. From time immemorial, people have seen blood flow from the body and with it, the life of the individual. The ancients thus presumed that blood carries a mysterious "vital force," and Roman gladiators drank it to fortify themselves for battle. Even today, we become especially alarmed when we find ourselves bleeding, and the emotional impact of blood is enough to make many people faint at the sight of it. From ancient Egypt to nineteenth-century America, physicians drained "bad blood" from their patients to treat everything from gout to headaches, from menstrual cramps to mental illness. It was long thought that hereditary traits were transmitted through the blood; people still use such unfounded expressions as "I have one-quarter Cherokee blood."

Scarcely anything meaningful was known about blood until the first microscopes revealed the blood cells. Blood is a uniquely accessible tissue for study, yet most of what we know about it dates only to the last 50 years. Recent developments in hematology–the study of blood–have empowered us to save and improve the lives of countless people who would otherwise suffer or die.

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