The immune system likes familiarity

The immune system likes familiarity. If it's never seen a substance before it will attack. If you put type B blood into someone with type A, their immune system will recognize the B markers as foreign and attack the transfusion. You can't give AB to anyone except for another person with AB blood; both type As and Bs will react because it contains something their immune systems have never seen before.

O is special because it's the absence of both A and B cell surface markers. There's nothing on the cells that's not on all human blood cells. The recipient's immune system won't react.