The correct option is O.
Explanation
The ABO blood group system classifies human blood based on the presence or absence of two specific antigens, Antigen A and Antigen B, on the surface of Red Blood Cells (RBCs). Transfusion compatibility is determined by the interaction between the donor's antigens and the recipient's antibodies found in the plasma.
Detailed Analysis
- Blood Group O (Universal Donor): Individuals with blood group O lack both Antigen A and Antigen B on the surface of their RBCs. Since these antigens are absent, the recipient's immune system (specifically antibodies Anti-A or Anti-B) does not recognize the transfused cells as foreign. Consequently, no agglutination (clumping) occurs, allowing group O blood to be transfused to individuals of any blood group (A, B, AB, or O).
- Blood Group AB (Universal Recipient): Individuals with blood group AB have both Antigen A and Antigen B on their RBCs but lack both Anti-A and Anti-B antibodies in their plasma. This absence of antibodies allows them to receive blood from any group.
- Blood Groups A and B: These groups possess specific antigens (A and B, respectively) and corresponding antibodies. They are not universal donors as their specific antigens would trigger an immune response in incompatible recipients.
Note: While "O" is generally termed the universal donor, specifically O-negative (O-) is the precise universal donor type because it also lacks the Rh antigen.
Key Takeaway
Blood Group O is the universal donor because its RBCs have no surface antigens (A or B) to trigger an immune reaction. Blood Group AB is the universal recipient because its plasma contains no antibodies against A or B antigens.