The accurate grouping of blood is very important when it comes to having a blood transfusion. If blood is given to a patient that has a blood type that is incompatible with the blood type of the blood that the patient receives, it can cause intravenous clumping in the patient's blood which can be fatal. The patient’s body can start producing antibodies that attack the antigens on the blood cells in the blood that was given to the patient, causing reaction and rejection.
For example, a patient who is blood group B has naturally occurring Anti-A antibodies in their blood plasma. If this (blood group B) patient receives blood group A red cells, the Anti-A antibodies in the plasma of the patient will cause the blood group A red cells to clump intravenously (within the veins), which is life threatening.