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January is National Blood Donor Month, which makes this a good time to learn more about blood types.
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.
Ideally, blood transfusions are performed with donated blood that's an exact match for type and Rh factor. But what happens in an emergency, when there isn't time or opportunity to check?