A1B? I've never heard of this referred to as a blood group, do you mean AB? The main advantage of this blood group is that it can receive blood donations from any blood group, hence its known as the universal receiver. It can receive any type of blood because it has not antibodies present on its red blood cells, which means there's no danger of the blood clotting when mixed with other blood groups and their antigens. I don't really know of any disadvantages...
As for the positive part, as far as I know there's only a disadvantage here if the person with blood group AB+ is male, and his partner is - with any other blood group, and they have a baby. In this instance the baby may have inherited the fathers positive aspect, whereas the mother has the negative attribute. If the baby and mothers blood come into contact during child birth, this could cause the baby's blood to clot, and in some circumstances the baby could die. The disease is known as hemolytic disease if I remember right. Its a lot more complicated than that, but I'm not an expert and studied this a while ago. If anyone else can add anything that would be great.
As for the positive part, as far as I know there's only a disadvantage here if the person with blood group AB+ is male, and his partner is - with any other blood group, and they have a baby. In this instance the baby may have inherited the fathers positive aspect, whereas the mother has the negative attribute. If the baby and mothers blood come into contact during child birth, this could cause the baby's blood to clot, and in some circumstances the baby could die. The disease is known as hemolytic disease if I remember right. Its a lot more complicated than that, but I'm not an expert and studied this a while ago. If anyone else can add anything that would be great.