Why can't a person with blood type A safely receive a transfusion of blood type B?
A person with A blood group naturally posseses antibodies against B group blood.
ABO blood group is determined on the basis of antigens present on the surface of RBCs, but there are possibilities of presence of antibodies in plasma as well.
In blood of A type, there is antigene A on RBCs and antibody b in plasma. If blood type B is introduced in a person of blood group A, all his antibody b will attack the RBCs of donated blood. This could be explained by the fact that antigen B present on RBCs of donor are recognised as foreign antigen, hence attacked. Transfused blood become coagulated, the clots may clog arterial passages to cause havoc inside body.