Which two blood groups should not marry?

In general: If you have type A blood, you should only receive types A or O blood. If you have type B blood, you should only receive types B or O blood. If you have type AB blood, you can receive types A, B, AB, or O blood.

What blood types are not compatible?

People with type A blood will react against type B or type AB blood. People with type B blood will react against type A or type AB blood. People with type O blood will react against type A, type B, or type AB blood. People with type AB blood will not react against type A, type B, type AB, or type O blood.