The four different groups of blood are classified as group A, group B, group AB and group O, according to the American Red Cross. Each group contains two blood types, a plus type and a minus type. For example, blood group A includes type A+ and A-.
The lettering system used to divide blood groups refers to the type of antigens on red blood cells and antibodies in the plasma, explains the American Red Cross. For example, group A blood has only A antigens on the red blood cells, while red blood cells in group AB blood have both A and B antigens.
The plus or minus that divides blood groups into types indicates the presence or absence of another antigen called the Rh factor. Blood types containing the Rh factor contain a plus sign, while those without the Rh factor are indicated by a minus sign. For example, blood type B+ contains both B antigens and Rh factor antigens, while B- type blood contains only B antigens.
Blood type is inherited and is used by doctors to determine the appropriate type of blood to use in transfusions for patients. The most common blood type is O+, which has no A or B antigens but does contain Rh factor antigens and both A and B antibodies in the plasma.