In the context of human blood transfusion, which one of the following blood groups is commonly referred to as the universal donor type for red blood cells because it can generally be given safely to recipients of most other ABO blood groups in emergencies?

Introduction / Context:
Blood transfusion relies on compatibility between donor and recipient blood types to prevent dangerous immune reactions. The ABO and Rh blood group systems are especially important. In emergency situations, one particular blood type is often considered a universal donor for red blood cells because it lacks certain antigens that could trigger a reaction in most recipients. This question checks whether you know which blood group is given this universal donor label in basic medical general knowledge.

Given Data / Assumptions:

• The focus is on human blood groups in the ABO and Rh systems.
• We are asked about the universal donor type for red blood cells.
• Options include combinations such as O negative, O positive, AB positive, AB negative, and A positive.
• We assume standard textbook teaching, which simplifies compatibility for competitive exam purposes.
• We are talking about red cell transfusions, not plasma transfusions.

Concept / Approach:
In the ABO system, red blood cells of group O do not carry A or B antigens on their surface. In the Rh system, negative indicates the absence of the Rh D antigen. A person with O negative blood therefore has red cells that lack A, B, and D antigens. When these red cells are transfused into most recipients, there are no major ABO or Rh antigens to be attacked by the recipient antibodies, which reduces the risk of a severe immediate reaction. For this reason, O negative is called the universal donor for red cells in emergency practice. However, it is best to cross match and use the exact blood group whenever possible.

Step-by-Step Solution:
Step 1: Recall that blood group O has no A or B antigens on red cell surfaces. Step 2: Remember that Rh negative blood lacks the D antigen. Step 3: Combine these facts to see that O negative red cells carry neither A nor B nor Rh D antigens. Step 4: Understand that recipients may have antibodies against A, B, or D antigens if those are foreign to them. Step 5: Recognise that since O negative red cells have none of these antigens, they are unlikely to be attacked by such antibodies in most recipients. Step 6: Evaluate other blood groups and note that they carry at least one of these antigens and therefore cannot be universal donors. Step 7: Conclude that O negative is regarded as the universal donor blood group for red blood transfusions.

Verification / Alternative check:
Medical textbooks and blood bank guidelines frequently refer to O negative units as emergency universal donor blood, especially when there is no time for full compatibility testing. Charts of ABO and Rh compatibility show that O negative red cells can be given to almost any ABO Rh recipient, although small risks and minor antigen differences still exist. In contrast, AB positive is known as the universal recipient type for red cells. These standard descriptions confirm that O negative is the correct answer.

Why Other Options Are Wrong:
O positive (O+): Lacks A and B antigens but carries the Rh D antigen, which can cause reactions in Rh negative recipients. AB positive (AB+): Has A, B, and D antigens and is considered the universal recipient for red cells, not the universal donor. AB negative (AB−): Has both A and B antigens and therefore cannot safely donate red cells to most other ABO types. A positive (A+): Carries A and D antigens and can only be transfused safely into compatible recipients, not all groups.

Common Pitfalls:
One common confusion is between universal donor and universal recipient. AB positive individuals can receive red cells from any ABO group because their immune system usually does not form anti A or anti B antibodies, but their red cells are poor donors. O negative individuals are the opposite: their red cells are widely compatible donors, but they themselves can receive only O negative blood. Remembering this contrast helps avoid mixing up the roles of O negative and AB positive in exam questions.

Final Answer:
O negative (O−) is commonly referred to as the universal donor blood group for red blood cells.