Blood is unique among other body tissues due to its liquefied nature. As a result of this curious researchers could obtain a blood sample and experiment with it more conveniently than any other body tissues. Some of the very early experiments involved blood transfusion from animal to animal and transfusion from animal to human. However, most of these experiments ended up with detrimental outcome to the recipient due to severe allergic reactions.

In 1901 the Austrian Scientist Karl Landsteiner, for the first time in the history, postulated the concept of blood groups. He obtained blood samples from several individuals and separated red cells and serum. Once separated, he mixed the red cells of one individual with serum of another simulating a transfusion in vitro. He demonstrated that the said mixing resulted in a visible agglutination reaction in some pairs while others did not do so. Finally he concluded that this observation is due to the existence of blood and the ABO blood group system was discovered. For this historical discovery Karl Landsteiner was awarded with the Nobel Prize for Medicine in 1030.

His birthday falls on 14th of June and World Health Organization selected this to celebrate “World Blood Donor Day” to pay tribute and motivate voluntary non remunerated blood donors.

Landsteiner’s postulation about ABO blood group system has been scientifically proven and studied in detail. According to the current understanding differences in the molecular structure of glyco-proteins known as antigens found on the surface of the red cells determine the blood group. For example in ABO blood Group system there are two antigens known as A antigen and  B antigen and they give rise to following blood groups.

In 1940 Karl Landsteiner and Alexander Wiener discovered another red cell antigen. It was identified as Rhesus factor or Rhesus antigen as it was discovered while they experimented using the red cells obtained from Rhesus Monkey (Rhesus macaque). Rh antigen can be found in more than 90% of the individuals they are typed as Rh-positive. Individuals who do not have the Rh antigen on their red cells are typed as Rh-negative. Accordingly the 4 blood groups in the ABO system can be subdivided further as follows.

The type of antigen present on your red cells and the resulting blood group is genetically determined. If you have the relevant gene responsible for the formation of a particular antigen inherited form your parents your blood group is determined accordingly.

Where there is an antigen there should be an antibody

Our immune system is in constant surveillance about the microbes invading our bodies. Various molecules formed by basic elements like carbohydrate and protein found on the outer surface of invading bacteria and viruses are recognized by the immune system as foreign. Since they stimulate the immune system such foreign substances are known as antigens. Once the immune system recognize a foreign antigen a protein that can react with those antigens are produced by the immune system. These proteins are known as antibodies and antibodies helps to neutralize the invading pathogens if the body expose to same pathogen in a subsequent occasion.

While this antibody production naturally occurs in healthy people immunization enhances the antibody production even before the natural pathogens invade the body. For example when you are vaccinated against polio the immune system produces antibodies against polio virus and even if you exposed to the virus subsequently the antibodies in your blood inactivate the virus and you are protected against polio.

Interestingly the immune system produces antibodies only against foreign antigens. Structures similar to A and B antigen found on the red cell are abundant in the microbes and food particles we expose in day today life. As a result of this our immune system produces antibodies against them and they are called A antibody and B antibody. However, the immune system makes surenot to produce an antibody against an antigen inherited on your red cells since it is not foreign. The production of A and B antibodies naturally occurs around 3 to 4 months after birth and the possible combination of antigen and antibodies are as follows.

Compatibility of blood donor and recipient

During a blood transfusion red blood cells obtained from a donor is given to a vein (a blood vessel that carry blood towards the heart) of the patient or recipient through a blood transfusion set. After the transfusion the donor’s red cells dissolves in the recipient’s plasma and circulates all over the recipient’s body. Since our immune system is on constant surveillance on the foreign substances entering into the blood circulation transfused red cells are also subjected to this surveillance. The immuno-globulins or antibodies in the recipient’s plasma would react against the antigens of donor red cells if they contain a foreign antigen. For example group A blood cannot be given to a group B patient since antigen A on donor red cells reacts with A antibody in the recipients plasma giving rise to a transfusion reaction. Furthermore, prior to a safe transfusion we must consider about the antigen on donor’s red cells and antibodies in the recipient’s plasma. In the transfusion laboratory this is determined by a compatibility testing. Compatible donors and recipients are marked in the following table,

Group O red cells are universal donors since they do not have A or B antigens on them they can escape the immune surveillance of any of the recipients. Group AB individuals are universal recipients since they do not have A or B antibodies in their plasma they do not mount a reaction against any of the ABO groups.