There are more than 20 genetically-determined blood group systems known today, with the ABO system being the most important given its vital role in the safe use of blood transfusions. It was discovered in 1901 by Austrian haematologist and physician Karl Landsteiner who won a share of the 1930 Nobel
Prize in for his work. The difference between each group is determined by the presence or absence of certain antigens on the surface of red blood cells, and the presence or absence of corresponding antibodies in the circulating blood plasma.
Blood group A: when A antigens are present on the surface of red blood cells and B antibodies are circulating in the plasma.
Blood group B: when B antigens are present on the surface of red blood cells and A antibodies are present in the plasma.
Blood group AB: when both A and B antigens are present on red blood cells and no A or B antibodies at all are in the plasma. People with AB blood are known as universal recipients because they can receive blood donations from any other ABO group.
Blood group O: sometimes known as “null” because you have neither A or B antigens on the red cells, but have both A and B antibodies in the blood plasma. Type O people are known as universal donors because their blood is compatible with any other ABO blood type.