There is a decrease of blood in human due to more bleeding or due to any disease then doctors demand to donate blood from any relative to patient to overcompensate blood. Before doing this they undergo matching the blood of both patient and the donor. If unmatched blood is transfused to patient, as reaching in its body, RBCs starts sticking together to form large clumps. It is called agglutination of blood. It ultimately leading to death of the patient. Carl Landsteiner observed that the blood of all human beings is not same and the blood group of the recipient and the donor should be same for a successful transfusion. RBCs and plasma have some specific proteins, due to their interaction agglutination of blood takes place. For this discovery Landsteiner awarded Nobel Prize. Antigen and Antibodies of Blood: Human blood has two types of proteins: A. Antigen or Agglutinogens: These are specific glycoproteins. These are present on membrane of RBCs. These are of two types: Antigen - A and Antigen - B. On the basis of antigen present on RBCs, Landsteiner divided human blood into four groups: � Blood group - A: A person having blood group A, has antigen A on RBC. � Blood group - B: Antigen - B is present on red blood corpuscles. � Blood group - AB: Both the antigens A and B are present on the RBCs of the person. � Blood group - O: There is no antigen on the RBCs of a person. B. Antibodies or Agglutinins: These are found in blood plasma. These are also of two types: anti - a and anti - b. The antigens which are not found on RBCs of human, there initially antibodies found in serum against them. For example, antigen B is not found on RBCs of human having blood group - A, hence its blood serum has antibody - a or antibody - b. Similarly antigen - A is absent on RBCs of a person having blood group B. Its serum contain antibody a or b. Both antigen A and B are present on RBCs of a person having AB blood group. No any antibody found in its blood serum. The person having blood group O lacks any type of antigen on RBCs but has both antibody - a and antibody b. It is clear from the above description that the blood of any of the person can not transfuse to patient. For this purpose first blood group testing is required. Person of blood group A can take the blood of A and O group and can donate blood to person of A or AB blood group. Similarly, the person of blood group B can take the blood of B and O group and can donate the blood to person having B or AB blood group. The person having O blood group can not take blood of any other blood group but can donate to any other blood groups. It is called universal donor. The person having blood group AB can take the blood from any of other blood groups but can donate only to person having blood group AB. Blood group AB is called universal recipient.