What is the logic of the transfusional compatibility concerning the ABO blood group system?
The transfusional compatibility for the ABO system takes into account the antigens show in the red blood cells of the donor and the antibodies that the recipient can make. Whenever the recipient is not able to produce antibodies against antigens of the red blood cells of the donor the transfusion is compatible.
So regarding ABO compatibility type A can donate to type A and to type AB. Type B can donate to type B and to type AB. Type AB can donate only to type AB. Type O can donate to all ABO types.