The blood type that could be used without danger of antibodies clumping its red cells is O-negative.

Here's why:

* O-negative blood lacks both A and B antigens on the surface of its red blood cells. This means it won't trigger an immune response from people with A, B, AB, or O blood types.

* O-negative blood also lacks the Rh factor. This makes it compatible with both Rh-positive and Rh-negative individuals.

Therefore, O-negative blood is considered the "universal donor" because it can be safely transfused into anyone without causing an adverse reaction.