Explain with reasons.

Person with 'O' blood group is considered as 'universal donor'.

Person with 'O' blood group is considered as 'universal donor' because such an individual can donate blood to a person having any other blood group. Blood types are determined by the presence or absence of certain antigens – substances that can trigger an immune response if they are foreign to the body. Since some antigens can trigger a patient's immune system to attack the transfused blood, safe blood transfusions depend on careful blood typing and cross-matching. Group O has neither A nor B antigens on red cells (but both A and B antibody are in the plasma) therefore it act as a universal donor of blood.