The four major blood types present in the human population include group A, group B, group AB and group O, according to the Red Cross. These blood type groups are based on blood antigens and antibodies.
Each red blood cell has zero, one or two antigens present, according to the Red Cross. The A and B surface antigens present on red blood cells determine blood type. Red blood cells without these antigens are indicative of type O blood. If a patient has only the A or only the B then they have type A or type B blood, respectively. If they have both A and B antigens present then they have AB blood.