Blood has to be arranged for a person undergoing a surgery (operation) or in the case of an accident or in case of persons suffering from thallasemia. This arrangement is to replace blood lost from the patient.
Injecting blood into the body from outside is called blood transfusion. Blood transfusion is successful only when the blood of donor (who gives blood) and of the recipient (who receives blood) match. Unmatched blood transfusion causes agglutination (clumping together) of red cells due to which the recipient may even die.
On the basis of types of proteins present in the blood, a system of blood groups known as ABO system having four blood groups named A, B, AB and O is recognized in human blood. Antigens present on membrane of RBC of transfused blood is counteracted by antibodies present in the plasma of recipient.
The persons with blood group O can donate blood to all and so O group is called universal donor and AB group can receive blood from donors of all blood groups and is called universal recipient.