Which transfusion reaction is classified as a delayed reaction?

Transfusion-associated graft-versus-host disease (TA-GvHD) is classified as a delayed transfusion reaction primarily because it typically occurs days to weeks after the transfusion, rather than immediately. TA-GvHD is a rare but serious complication that arises when transfused immune cells from the donor recognize the recipient's tissues as foreign and attack them. This condition is particularly concerning in immunocompromised patients, where their ability to combat such an immune response is severely diminished.

In contrast, the febrile non-hemolytic reaction occurs shortly after the transfusion, usually within hours, as a response to leukocyte-derived cytokines. Acute immune hemolytic reactions occur immediately or within the first 24 hours post-transfusion due to ABO blood group incompatibility. Allergic reactions, which may include symptoms like hives or anaphylaxis, also typically present during or shortly after the transfusion. Thus, TA-GvHD stands out due to its delayed onset and mechanisms, distinguishing it from the other options presented.