In the ABO blood group system, which blood group is traditionally known as the universal donor for red blood cell transfusions?

Introduction / Context:
The ABO blood group system is a fundamental topic in human physiology and medical general knowledge. It divides blood into groups A, B, AB, and O based on specific antigens on red blood cells. Two classic terms often asked in exams are universal donor and universal recipient. This question asks you to identify which blood group is considered the universal donor for red blood cell transfusions, a standard fact in many textbooks and competitive exams.

Given Data / Assumptions:

• The ABO blood groups considered are A, B, AB, and O.
• The question refers to the traditional concept of a universal donor.
• We assume school level understanding that focuses mainly on ABO grouping, with a note about Rh factor.
• The emphasis is on red blood cell transfusion compatibility, not on plasma transfusion.

Concept / Approach:
In the ABO system, blood group O lacks A and B antigens on the surface of red blood cells. This means that group O red cells are less likely to be attacked by antibodies against A or B in the recipient's plasma, because they do not carry those antigens. Therefore, group O, particularly the Rh negative subtype O negative, is traditionally described as the universal donor for red blood cells. In contrast, group AB individuals have both A and B antigens and can safely receive any ABO type, making them universal recipients rather than donors.

Step-by-Step Solution:
Step 1: Recall that blood group A has A antigens on red blood cells and anti B antibodies in plasma. Step 2: Blood group B has B antigens and anti A antibodies in plasma. Step 3: Blood group AB has both A and B antigens and no anti A or anti B antibodies in plasma, so it is the universal recipient, not donor. Step 4: Blood group O has no A or B antigens on its red blood cells but has both anti A and anti B antibodies in plasma, so its red cells can be given to individuals of other ABO groups. Step 5: Therefore, group O, especially O negative when the Rh factor is also considered, is known as the universal donor for red blood cell transfusions.

Verification / Alternative check:
Most textbooks and exam guides present a simple table stating that group O negative is the universal donor and group AB positive is the universal recipient. The negative Rh factor is important because Rh positive blood cannot always be given safely to Rh negative recipients. For exam questions that list only A, B, AB, and O without mentioning Rh, O is still taken as the universal donor. This common and consistent description confirms that blood group O (especially O negative) is the correct answer.

Why Other Options Are Wrong:
Blood group A is wrong because its red cells carry A antigens and can be attacked by anti A antibodies in recipients with blood group B or O. Blood group B is also wrong, as its red cells carry B antigens and can be attacked by anti B antibodies in groups A or O. Blood group AB is incorrect since AB cells have both A and B antigens and can be given safely only to AB recipients; however, AB individuals can receive blood from any ABO type, making them universal recipients instead of donors.

Common Pitfalls:
A common pitfall is to confuse universal donor with universal recipient and mistakenly choose AB instead of O. Another misunderstanding arises when students ignore the Rh factor and oversimplify real clinical transfusion practice. In real medicine, cross matching and Rh typing are essential before transfusion. For exam purposes, however, you should remember the key association: O negative is the universal donor for red blood cells, and AB positive is the universal recipient.

Final Answer:
The correct choice is Blood group O (especially O negative), which is traditionally regarded as the universal donor blood group for red blood cell transfusions.