Which blood group is the universal donor in the ABO system?

• A. A positive
• B. O negative
• C. AB positive
• D. B negative

Answer: (B)

The universal donor idea applies to red blood cell compatibility. Type O negative red blood cells lack both A and B antigens and also lack the Rh antigen. Because they don’t present A or B antigens or the D antigen, they are unlikely to trigger immune reactions in recipients with any ABO or Rh type. That’s why O negative is used when the patient’s blood type isn’t known or when rapid transfusion is needed. The other types carry A or B antigens or the Rh antigen, which means they can cause reactions in recipients with different blood types.