Final answer:
The blood type is O when neither anti-A nor anti-B serum causes agglutination of donor cells, as type O blood lacks both A and B antigens on erythrocytes, making it suitable for transfusion to any ABO blood type.
Step-by-step explanation:
When neither anti-A serum nor anti-B serum cause agglutination (clumping) of donor cells on a slide, the blood type is O. This is because people with type O blood lack both antigens A and B on their erythrocytes. However, they do have both anti-A and anti-B antibodies circulating in their blood plasma. Because of the absence of these antigens, type O blood can be transfused into individuals with any ABO blood type, which is why individuals with this blood type are considered universal donors.