D.T. has her blood shipped thousands of miles from South Africa all the way to a hospital in Baltimore. Although there are millions of donors for some types of blood, D.T.’s blood type is so rare that there is not a single known donor in the U.S. that can match it. To find a suitable blood donor, D.T.’s physicians turned to the American Rare Donor Program (ARDP). D.T. is an African-American woman in her 40s with sickle cell disease. She has a long medical history that includes episodes of acute pain and life-threatening anemia requiring chronic red cell transfusions. Over time, she has developed antibodies to red cell antigens that she lacks. And she lacks many antigens — her phenotype is C-, E-, K-, S-, Fy(a-), and hrB-. While the ABO and Rh blood group systems are the most well-known, several others exist. Systems such as Duffy, Kidd, Kell, Diego, Colton, P, Xg and Indian are some of the nearly two dozen systems recognized by the International Society of Blood Transfusion, the organization that has helped to standardize them. These systems — which were often named after the patients first discovered to have these rare types of blood — identify nearly 600 red blood cell antigens. Generally speaking, a person’s blood type can be considered rare if it differs from the majority of the population in one of three ways: either the person has red blood cell antigens not commonly found in most of the population, lacks antigens that are common in most of the population, or is deficient in IgA — the immunoglobulin that may help fight infections. Strictly speaking, a person’s blood type is rare if only one of 1,000 people has or lacks the same red blood cell antigens. A blood type is very rare if the match is one in 10,000. D.T. lacks so many antigens that makes her blood unique even within the rare donor community. For D.T., there simply was not a match in the entire country.