When the Food and Drug Administration (FDA) lifted a decades-old blood donor ban last year, Hop Tran Cao, M.D., immediately rolled up his sleeve.

“I have wanted to donate for a very long time but had previously never been permitted because I grew up in France during the ‘mad cow disease years’ in Europe,” says Tran Cao. “I signed up to donate at the MD Anderson Blood Bank as soon as I heard about the change in eligibility.”...