In the beginning of the movie Grace says that she needs to donate blood because she has a rare blood type, AB positive. In reality, blood banks don't want AB blood because they can only give it to a very small amount of people, while people with AB blood are known as "universal recipients" and can accept any type of blood, whether it be A, B, AB, or O. Blood banks want O blood, the "universal donor" because they can give it to any patient.