If a patient has type AB blood and the hospital is out of AB blood, which blood type (A, B, or O) can she safely receive, and why?

What the question is really testing

You are matching donor red blood cell antigens (A and/or B, or neither) with the recipient’s antibodies. A transfusion reaction happens when the recipient has antibodies that bind to antigens on the donor’s red blood cells.

AB blood: antigens present and antibodies absent

• Type AB red blood cells have both A and B antigens.
• Because the body sees both antigens as “self,” a type AB person has:
• No anti-A antibodies
• No anti-B antibodies

Checking each available donor type (A, B, O)

Think in terms of what antigens are on the donor red blood cells:

• Type A donor cells have A antigen. An AB recipient has no anti-A, so this is safe.
• Type B donor cells have B antigen. An AB recipient has no anti-B, so this is safe.
• Type O donor cells have neither A nor B antigens, so there is nothing for anti-A or anti-B to bind to. This is safe for an AB recipient.

Why AB is called the universal recipient (for red blood cells)

In the ABO system, type AB recipients lack both major ABO antibodies, so they can receive RBCs from A, B, AB, or O donors.

One important real-world note

This answer is for ABO compatibility of red blood cells. In real transfusions, hospitals also match Rh factor (positive/negative) and do crossmatching, which can change what is considered “safest,” especially for Rh-negative patients.