Blood consists of plasma and cellular elements .A person’s blood is determined by the type of protein present on the surface of RBC ‘s called antigen . The antibodies present in the plasma is determine by the type of antigen present on the surface of RBC’s. Blood needs to be cross matched before transfusion because if a wrong type of blood group is transfused in the human beings it results in complications and thus leads to clotting of blood . So the blood group must be checked before transfusion.