Question

In the ABO system, blood group O is characterised by

• A. absence of both antigen A and antigen B
• B. presence of antigen O
• C. presence of antigen A and absence of antigen B
• D. presence of both antigen A and antigen B

Answer 1

The correct option is A absence of both antigen A and antigen B

The plasma of different individuals contain two natural antibodies (proteins produced in response to antigens) namely A and B with respect to blood group. If an antigen is mixed with its corresponding antibody then it leads to agglutination (clumping). Hence, blood groups with antigen A on its surface should not contain antibody A in its plasma. People with blood group O have no A and B antigens on their surface but have A and B antibodies in the plasma.

$\begin{array}{cccc}\text{Blood group}& \text{Antigens on RBCs}& \text{Antibodies in Plasma}& \text{Donor's Group}\\ \text{A}& \text{A}& \text{anti-B}& \text{A,O}\\ \text{B}& \text{B}& \text{anti-A}& \text{B,O}\\ \text{AB}& \text{A,B}& \text{nil}& \text{AB, A, B, O}\\ \text{O}& \text{nil}& \text{anti-A, B}& \text{O}\end{array}$