Blood group O individuals do not have either A or B antigens on the surface of their RBCs. But their blood serum contains IgM anti-A and anti-B antibodies against the A and B blood group antigens. Therefore, an individual of blood group O can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group, i.e., A, B, O or AB. They are known as universal donors.