When your immediate blood relative needs a blood transfusion, I believe that most people will be obliged: “We are his relatives, use our blood!” Such scenarios are also common in film and television, but this “help” is often just a sensationalist scenario designed in film and television, which is counterproductive in real life. Blood transfusions between blood relatives can lead to a range of immune diseases, including the extremely dangerous TA-GVHD (transfusion-associated graft-versus-host disease). Under normal circumstances, the population incidence of TA-GVHD is only 0.1% to 1%, while the closer the blood relationship, the higher the incidence, which increases to 10-20 times between first-degree relatives. Even though the incidence is low, no effective therapy has been found, and once it occurs, the mortality rate is as high as 90%. Blood relatives mainly refer to the same ancestor, relatives who have the same part of genetic material, and blood donors and recipients who are related are more likely to have similar HLA (human leukocyte antigens) in their blood cells. The main basis for the occurrence of TA-GVHD is the identity and homology of the HLA hemiploidy of the blood recipient and the blood donor. The defenders of the recipient’s immune system identify the donor as friendly based on its antigens, thus allowing the imported lymphocytes to survive, clonally proliferate and activate in the recipient and later attack the recipient instead, thereby severely damaging the digestive and hematopoietic systems and triggering graft-versus-host disease, which is extremely lethal. The disease starts suddenly, progresses rapidly, and is so severe that doctors sometimes do not even have time to make a diagnosis. In the United States, transfusion-associated graft-versus-host disease rarely occurs in transfused patients, while it occurs more frequently in Japan, probably due to the presence of highly homozygous genes in the Japanese population. There are no accurate statistics on the incidence of graft-versus-host disease in China, estimated at 1 in 10,000, but the incidence can reach 1% in immunodeficient populations, and the disease is up to 10 to 20 times more common in first-degree relatives (including parents, siblings, and children) compared to unrelated blood donations. Therefore, direct blood transfusions between blood relatives should be avoided as much as possible to avoid morbidity.