Do you know your blood type or that of your close family? If you do, it might surprise you to hear that an awful lot of people have no idea, and even less know how compatible their type is with others if it were to come to an emergency. As this chart using American Society of Hematology information shows, there can be very big differences in how many types your body would accept in the case of a transfusion.
If you have the type AB+, you have the luxury of being able to accept all blood types, where as somebody with O- are severely restricted, only actually able to receive O- blood making for very high demand for O- donors. As stated by the source: "Safe, reliable source of blood is critical to providing effective blood products to recipients, blood banks are dependent on the altruistic voluntary donations of citizens. As a result, blood banks place great emphasis on making the donation process pleasant, convenient, and as safe as possible for donors."