January is National Blood Donor Month, which makes this a good time to learn more about blood types and the need for blood donation.
Nearly 16 million blood components are transfused each year in the U.S., according to the American Red Cross. Daily needs include 29,000 units of red blood cells, 5,000 units of platelets and 6,500 units of plasma.
People need a blood transfusion for many reasons. Some may need blood during surgery. Others depend on it after an accident or because they have a disease that requires blood components.
Blood donation makes all this possible. There is no substitute for human blood. All transfusions use blood from a donor.
Ideally, blood transfusions are performed with donated blood that's an exact match for type and Rh factor. There are four blood types: A, B, AB and O, each differentiated by specific proteins called "antigens." And blood also is classified by the rhesus, or Rh, factor — negative or positive.
But what happens in an emergency, when there isn't time or opportunity to check? For emergency transfusions, Type O-negative is the variety of blood that has the lowest risk of causing serious reactions for most people who receive it. Because of this, it's sometimes called the universal blood donor type.