Final answer:
Blood type O-negative is called the 'universal donor' because it lacks A and B antigens as well as the Rh antigen, making it compatible with all other blood types and preventing immune system reactions during transfusions.
Step-by-step explanation:
Blood type O-negative is referred to as the universal donor because it can be safely given to individuals of any blood type without causing any adverse immune response. Type O blood lacks A and B antigens on the red blood cells, which means it is not recognized as foreign by the recipient's immune system, preventing antibody reactions. Furthermore, the negative Rh factor (Rh-) of O-negative blood means it lacks the Rh antigen, which would otherwise risk triggering an immune response in Rh-negative recipients. Hence, O-negative blood is the safest choice for transfusions when the blood type of the recipient is unknown or in emergency situations where there is no time for blood type matching.