What is a blood transfusion reaction?

Many things. The person receiving blood may react to the blood or the tranfusion process. This may be as simple as fever or rash, or may involve antibodies reacting to proteins in the blood being transfused.
Immunologic or not. Causes for transfusion reactions may be immune related or not, also they can be acute (less than 24 hours) or delayed. These are some: hemolytic; febrile, non-hemolytic; bacterial contamination (septic shock); anaphylaxis; transfusion-related acute lung injury (trali); transfusion-associated circulatory overload (taco); graft vs. Host disease (ta-gvhd); iron overload; infections.

Related Questions

During a blood transfusion if you get a fever and no other problem is this a dangerous transfusion reaction?

Usually no. If the only adverse reaction is a fever then the reaction is unlikely to be dangerous/significant - patients recieving transfusions are sometimes pre-treated with tylenol (acetaminophen) and Benadryl to prevent these minor reactions from being too symptomatic. Read more...
Maybe. If there are no other symptoms it may be a reaction to something else. However, your physicians can easily investigate the presence/absence of a reaction, and the transfusion should be stopped at the first sign of one. Read more...