What is a risk associated with blood transfusions after the first 24 hours?

Alloimmunisation is a significant risk associated with blood transfusions that can emerge, particularly after the first 24 hours following a transfusion. This process occurs when the recipient's immune system recognizes the foreign antigens present on the transfused red blood cells as foreign and mounts an immune response against them. As a result, the individual can develop antibodies that will react against future transfusions of blood with similar antigens, complicating any subsequent transfusion needs by decreasing compatibility and increasing the risk of hemolytic reactions. This phenomenon highlights the risk of generating sensitization to red blood cell antigens, particularly with multiple transfusions over time, which can be especially concerning for patients requiring chronic transfusions or those with complex medical histories.

The other options, such as mild allergic reactions, transfusion-associated circulatory overload, and febrile non-haemolytic reactions, do not typically arise specifically after the first 24 hours. Mild allergic reactions often occur during or shortly after the transfusion. Transfusion-associated circulatory overload usually presents soon after transfusion, particularly in patients who are vulnerable, such as those with heart failure. Likewise, febrile non-haemolytic reactions usually occur within the early hours of a transfusion and not