Immunological transfusion reactions more than often lead to an activation of the complement proteins and mononuclear cells, inducing a haemolysis from which stem the observed clinical symptoms. In the case of incompatibility, the alloantibodies can lead to an immediate reaction, taking place in the first few minutes or, in the case of a delayed reaction, arising after 24 hours. A standardized clinical and biological evaluation is necessary in order to confirm the diagnosis and to assess the consequences of the antigen-antibody conflict.