In human blood grouping, which blood group is commonly known as the universal donor for red blood cell transfusions?

Introduction / Context:
Blood transfusion safety depends on matching blood groups correctly. The ABO and Rh systems are especially important. Some blood groups can donate red blood cells to a wider range of recipients than others. In basic biology and medical entrance exams, you are frequently asked which group is called the universal donor and why. This question focuses on that concept in the context of red blood cell transfusions.

Given Data / Assumptions:

• The question refers to the universal donor concept for red blood cells, not for plasma.
• Options include AB negative, O negative, AB positive, and O positive.
• We assume knowledge of ABO antigens and Rh factor on red cells.

Concept / Approach:
In the ABO system, type O blood lacks A and B antigens on the surface of red blood cells. In the Rh system, Rh negative blood lacks the Rh D antigen. When transfusing red blood cells, the main concern is that donor antigens should not be attacked by recipient antibodies. O negative red cells have no A, no B, and no Rh D antigens, so they are least likely to be recognized as foreign by recipients of any ABO and Rh type. For this reason, O negative is commonly referred to as the universal donor for red blood cells. However, for plasma transfusions, the universal donor concept is different, and compatibility rules are more complex in modern practice.

Step-by-Step Solution:
Step 1: Recall that type O red blood cells do not express A or B antigens on their surface. Step 2: Understand that Rh negative red cells do not express the Rh D antigen. Step 3: Recognise that O negative red cells therefore lack A, B, and Rh D antigens, minimizing immune reactions when given to recipients of different blood groups. Step 4: Identify O negative as the blood group commonly called the universal donor for red blood cell transfusions.

Verification / Alternative check:
Transfusion medicine guidelines list O negative red blood cells as the preferred choice in emergencies when the recipient blood group is unknown, especially for women of childbearing age. This is because the absence of A, B, and Rh D antigens reduces the chance of acute hemolytic transfusion reactions. Educational charts and posters often label O negative as the universal donor and AB positive as the universal recipient for red blood cells, reinforcing this idea in clinical training.

Why Other Options Are Wrong:

• AB negative: Red blood cells of AB group carry both A and B antigens, so they can only be given safely to recipients who do not have antibodies against these antigens.
• AB positive: This group has A, B, and Rh D antigens and is considered a universal recipient for red cells, not a universal donor.
• O positive: Although O positive lacks A and B antigens, it has the Rh D antigen and cannot be safely given to Rh negative recipients without risk of sensitization.

Common Pitfalls:
Students sometimes confuse universal donor and universal recipient or forget to consider the Rh factor. Another common mistake is to assume that the universal donor concept applies equally to plasma and red cells when in fact plasma compatibility is different. For exam questions about universal donor, you should think specifically about O negative red blood cells and the absence of A, B, and Rh D antigens.

Final Answer:
The blood group commonly known as the universal donor for red blood cell transfusions is O negative (O−).