Today blood transfusions are quite a commonplace event. They contribute to saving many human lives. However, before the discovery of blood groups by Karl Landsteiner in 1930, for which he was awarded the Nobel Prize, blood transfusions often lead to complications and even death.

The composition of the blood of one person is not exactly the same as that of another person. If the blood of the wrong type is transfused into a patient, it can lead to a clinical condition called blood clumping. Blood clumping is an immunological reaction which occurs when the receiver of a blood transfusion has antibodies against the donor blood cells. This can clog blood vessels and stop the circulation of the blood to vital parts of the body.

Blood consists of several types of cells floating in a fluid called plasma. The red blood cells transport oxygen to body tissues. The white blood cells fight infection. The platelets help the blood to clot. The plasma itself contains salts and various kinds of protein molecules called antigens and antibodies. The antigens are located on the surface of the red blood cells, while the antibodies are in the blood plasma. Individuals have different types and combinations of these antigens and antibodies in their blood, giving rise to different blood groups. According to the ABO blood grouping system, which is the most well-known blood grouping system, there are four different kinds of blood groups: A, B, AB or O.

Blood group A

People with blood group A have A antigens on the surface of their red blood cells and B antibodies in their blood plasma.

Blood group B
People with blood group B, have B antigens on the surface of their red blood cells and A antibodies in their blood plasma.

Blood group AB
People with blood group AB, have both A and B antigens on the surface of their red blood cells and no A or B antibodies at all in their blood plasma.

Blood group O
People belonging to the blood group O, have neither A or B antigens on the surface of their red blood cells but have both A and B antibodies in their blood plasma.

Frequency of blood groups

Blood types are not evenly distributed throughout the human population. Overall, the O blood type is the most common, although in some areas, such as Sweden and Norway, the A group dominates. The A antigen is more common than the B antigen. Since the AB blood type requires the presence of both A and B antigens, the AB blood type is the rarest.

Rh factor

Many people also have a so called Rhesus factor, or Rh factor, on the red blood cell's surface. This is also an antigen and those who have it are called Rh+, while those who don't are called Rh-. It is named after the Rhesus Monkey, in which this antigen was first identified in 1940. A person with Rh- blood does not have Rh antibodies in the blood plasma. But a person with Rh- blood can develop Rh antibodies in the blood plasma if he or she receives blood from a person with Rh+ blood, whose Rh antigens can trigger the production of Rh antibodies. In order to avoid complications, the Rh factor also has to be taken into consideration while giving blood transfusions.

How is the blood group of a person determined?

To do this, the person's blood is mixed with three different reagents, each containing one of the three antibodies for A, B or Rh antigens. The three mixtures are then studied carefully for agglutination. This gives an indication of the presence or absence of A, B, or Rh antigens. Once this is known, it is easy to determine the blood group.

Who can receive blood from whom?

For a blood transfusion to be successful, the blood groups of the donor and the patient must be compatible. Of course any person can receive blood from another person having the same blood group, but in some cases a person can also receive blood of another blood group, or donate blood to a person with a different blood group. No complications will result in such cases if the person who is going to receive blood has a blood group that doesn't have any antibodies against the donor blood's antigens. The table below shows the compatibility of different blood groups.

Blood Group             Can give blood to               Can receive blood from

AB                               AB                                          AB, A, B, O

A                                 A and AB                                A and O

B                                 B and AB                                B and O

O                                 AB, A, B, O                             O

People with blood group O are called "universal donors" and people with blood group AB are called "universal receivers."