In the waiting room at the blood donation center I talked to two other blood donors and discovered something interesting.
One donor was a woman with blood type O negative, which makes her a "universal donor". Anyone with any blood type can be transfused with blood from an O- donor. The flipside is that she's the hardest to find blood for if she ever needs it, because people with blood type O- can be transfused only with blood from another O- donor.
Across from her sat a woman who was just the opposite. She was type AB, which makes her blood useful for fewer people who need blood transfusions, but makes it easier to find blood if she ever needs it. Donors who are type AB positive are "universal recipients" since they can accept blood from any blood type, but their donated blood can be used only for other AB+ patients.
When there are shortages of blood, which happens from time to time, especially around the major holidays when people are so busy, O- blood is the most critical supply to have, and O- patients are at the most risk.