By the time Louise Tilley got to the blood sample, it had already been puzzling scientists for more than 30 years. In 1972, a pregnant woman had her blood taken, and doctors noticed that her red blood cells seemed to lack a surface marker, known as an antigen, that everyone else seemed to have. No one knew why. These antigens are vitally important because they define blood groups. If you ever receive blood that is a mismatch to your own, with the wrong antigens, it could kill you.
Not long after Tilley started working for NHS Blood and Transplant in the early 2000s, a colleague suggested to her that she
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