Final answer:
To avoid transfusion reactions, blood donated to a B+ person must not have antigens A or B.
Step-by-step explanation:
In order to avoid transfusion reactions, blood donors must not have antigens A or B on their red blood cells when donating blood to a person with blood type B+. This is because the recipient's blood contains antibodies against A and B antigens, and if these antigens are present on the donated blood, the antibodies will react and cause clumping of the red blood cells, which can be life-threatening. Therefore, blood from a universal donor with type O blood, which does not have A or B antigens, can be transfused to a B+ person to prevent an immune response.