Blood group is tested by two types of sera i.e., anti-A (antibody-A) and anti-B (antibody-B). Persons with blood group O possess both antibodies in their plasma but have no antigens in their RBCs. So, RBCs of blood group ' O ' do not show clumping in any of the two sera. That's why, persons with blood group ' O ' are called universal donor and they can donate blood to a person with any type of blood group.