BLOOD GROUPS
When more then 40% of blood is lost within few hours by haemorrhage or so it has to be restored by intravenous infusion this is known as blood transfusion it is an ideal method as it not only restores bloodo volume but also erythrocytes which may survive for 120 days However great care must be exercised while selecting the person who donates blood (donor)
When an incompatible blood is transfused intravenously from donor to recipient (that is person who receives blood) antibodies (agglutinins) present in recipients RBC bring about agglutination (clumping )of erythrocytes due to the presence of antigen (agglutinogens) present in the donors blood this results in severe haemolysis of erythrocytes damage and blocking of blood capillaries and death
According to ABO system of blood grouping there can be four blood groups "A" B" AB" and "O"This classification is based on the presence of agglutinogen "A" B both (A and B) or none of them respectively as Landsteiner stated if a particular type of agglutinogen is present in the blood then the corresponding agglutinin is always absent and if a particular type of agglutinogen is absent in the blood then the corresponding agglutinin is always present thus the blood group A .B and O have the agglutinin beta alpha and both respectively following table summarizes the ABO blood group system 
Blood group "O" is known as universal donor because it does not contain any agglutinogen and hence can be donated to any group
Blood group "AB" is known as universal recipient because it does not contain any agglutinin and hence can receive blood of any group now a days the terms universal recipient and universal donor are regarded as misnomer before transfusion of blood to any patient direct cross matching must be done
ABO system follows multiple allele system and hence transferred to consecutive generations according to mendel "s law this is used for paternity test in medico legal cases
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