Persons with which ABO blood group are called universal donors because their red blood cells can usually be given to people of most other blood groups?

Introduction / Context:
In blood transfusion, matching donor and recipient blood groups is vital to prevent dangerous reactions. Some blood groups can donate to many others, while some can receive from many groups. The term universal donor applies to a particular ABO blood group whose red blood cells can be transfused to almost any recipient in emergency situations, especially when only the ABO system is considered. This is a very common question in basic biology and general science exams.

Given Data / Assumptions:
The question asks which blood group is known as the universal donor.
Options list blood groups AB, A, O, and B.
We consider only the ABO grouping and ignore Rh factor for simple exam level discussion.
Universal donor here refers to red blood cells rather than whole blood plasma.

Concept / Approach:
In the ABO system, blood group O has red blood cells that lack both A and B antigens on their surface. Therefore, when group O red blood cells are transfused to recipients with groups A, B, AB, or O, there are no A or B antigens present for the recipient anti A or anti B antibodies to attack. This makes group O especially versatile as a donor type, and it is often referred to as the universal donor, particularly O negative when the Rh factor is included. Group AB, by contrast, has both antigens and is known as the universal recipient, not donor.

Step-by-Step Solution:
Step 1: Recall that blood group O has no A or B antigens on the red blood cells. Step 2: Understand that absence of these antigens reduces the risk of reaction when O cells are given to recipients of different ABO groups. Step 3: Remember that due to this property, group O is labeled as the universal donor in basic theory. Step 4: Confirm that group AB is the universal recipient, not the universal donor, and select group O as the correct answer.

Verification / Alternative check:
Standard compatibility tables in textbooks and hospital charts show that O can donate to all ABO groups, especially O negative when Rh is also considered. These charts also show that AB can receive from all ABO groups, reinforcing the idea that O is the universal donor and AB the universal recipient. This provides a clear check against confusion between the two terms.

Why Other Options Are Wrong:
Blood group AB has both A and B antigens and can receive from all groups but would cause reactions if transfused into recipients with antibodies against A or B, so it is not the universal donor.
Blood groups A and B each carry one type of antigen and would be incompatible with some recipients, so they cannot be universal donors either.

Common Pitfalls:
Learners sometimes mix up universal donor and universal recipient and may wrongly choose AB for both. Others forget that the term is based on antigens present on red blood cells, not on antibodies in plasma. Remembering the simple rule that O gives to all and AB receives from all in the ABO system helps prevent these mistakes.

Final Answer:
Persons with Blood group O are called universal donors.