Why do people with some blood types need compatible transfusions?

The ABO system classifies blood by A and B antigens on red blood cells, and the Rh system adds another major factor, especially the RhD antigen. If someone receives blood with antigens their immune system sees as foreign, antibodies can attack the donor cells. That can cause fever, shock, kidney failure, or death in severe cases. The modern ABO blood group system was identified by Karl Landsteiner in 1901, a discovery that made safer transfusions possible. Type O negative is often called the universal red-cell donor, while AB positive is often called the universal recipient type for red cells.