Individuals belonging to the blood group O are universal donors because ?

Analyze the question: It asks why people with blood group O are called universal donors. Their blood can be given to people of any ABO group.

Concept: ABO blood groups are determined by two things:

• Antigens on the surface of red blood cells
• Antibodies in the plasma

| Group | Antigens on red cells | Antibodies in plasma | |-------|----------------------|----------------------| | A | A | anti-B | | B | B | anti-A | | AB | A and B | none | | O | none | anti-A and anti-B |

Solution:

• In a transfusion, the main danger is that the recipient's antibodies attack the donor's red cell antigens, causing agglutination (clumping).
• Group O red cells have no A or B antigens, so the recipient's anti-A or anti-B antibodies have nothing to attack. This is why O blood can be given to A, B, AB and O recipients.

Checking the options:

• A. Antigens are absent in their blood cells: Correct. This is the real reason.
• B. Highest number of humans: Being common has nothing to do with compatibility.
• C. No antibodies in their plasma: False. Group O plasma contains both anti-A and anti-B. (Group AB is the one with no antibodies.)
• D. Antigens easily destroyed: False. O cells simply never had A or B antigens.

Answer: A

Tip: Remember "O = zero antigens, so O gives to all." For the reverse, "AB has no antibodies, so AB receives from all" (universal recipient). Option C is a common trap because it mixes up the two ideas.