REACTS to a blood transfusion.An antigen is any SUBSTANCE the immune system can respond to.If the immune system encounters an antigen that is not found on the body's own CELLS, it will set off an attack to FIGHT that antigen. O blood types are unique in that they have no antigens.The universal DONOR blood type is O negativeO negative blood is considered the universal blood donor type because it is compatible with all A, AB, B, and O positive blood types.