Amismatch of blood types during a transfusion is dangerous because preformed antibodies in the recipient's blood will bind and clump (agglutinate) the donated cells antibodies in the donor's plasma will attack and kill the recipient's healthy blood cells white blood cells from the donor's blood cause inflammation clotting factors in the donor's blood will cause unwanted clots known as thrombus

The answer is  Preformed antibodies in the recipient's blood will bind and clump the donated cells.

Every person’s blood type is determined by the antigen on their blood cells. If a person is of blood group A, then they have antigen A in their blood cells. Additionally, the same person will have antibodies to the alternative antigen, i.e in our case the person with blood group A will have antibodies for antigen B. Therefore if a patient with blood group A is transfused with blood group B blood, then the antibodies in the patient will bind to the transfused blood cells and cause agglutination.

the answer is golgi apparatus