Why would the administration of incompatible blood lead to haemolysis?

Answer 1

Blood types are determined by specific proteins that are on the outside of red blood cells. As our immune system develops, we develop antibodies against all proteins that are not already known to be in our bodies.

So, if I'm Type B, when my immune system develops, it will note that I have plenty of Type B blood proteins floating around and it will not form antibodies against them. It, will, however, form antibodies against Type A blood proteins since I haven't got any.

If, then, 20 years later, I'm given a transfusion of Type A blood, my own antibodies will recognize the Type A cells as "foreign" and will attack them and destroy the cells (haemolysis).

Answer 2

Haemolytic transfusion reactions occur when the antibodies in the recipient patients blood reacts against the antigens in the donor’s blood which results in rapid intravascular haemolysis. This occurs because of ABO incompatibility. This then causes haemoglobinuria, Haemoglbinemia, disseminated intravascular coagulation, renal failure and cardiovascular collapse.