When Blood of the wrong antigen is introduced to the body (antigen is what the body uses to recognize the right blood type), the body will begin to fight the foreign bodies. The blood will begin to clump in the blood cells, and the body will begin the process of destroying the blood; a process called Hemolysis. Between the clots and immune reaction, hemolysis can most definitely kill you, so it needs to be addressed as emergency in a hospital.
