The Type O blood group is commonly called as universal donor because any blood group can receive the O blood group. The blood group O has no antigen. Due to the absence of antigen it doesn't agglutinate when come in contact with other blood groups in the plasma.
The agglutination reaction happens when opposite antigen and antibody reacts with each other. e.g.- Blood group A contains antigen A and antibody b and the blood group B contains antigen B and antibody a. Agglutination reaction occurs when the antigen A reacts with antibody a. As O blood group has no antigen, agglutination reaction doesn't occur.
