Blood Types

Blood type refers to the categorization of human blood based on the presence of genetic antigens, which are carbohydrates that attach to lipids or proteins. Antigens are categorized as A, B and Rh.

There are 4 differing blood groups which determined by the presence or absence of two antigens (A and B) on the red blood cells: Blood group A (A antigen on red cells, B antibody in plasma), group B (B antigen on red cells, A antibody in the plasma), group AB (A and B antigens on red cells but neither A nor B antibody in plasma), and group O (neither A nor B antigens on red cells, A and B antibody are in plasma).

So in addition to the A and B antigens, the third antigen is called the Rh factor -- is either present or absent and is referred to as either positive (+) or negative (-). This, along with blood groups, determine what blood type a person is. Typically, Rh negative blood is only given to Rh-negative patients while Rh positive blood or Rh negative blood may be given to Rh positive patients.