Question

In which one of the following combinations, ABO incompatibility is most common?

• A. Mother's group 'A' and father's group 'O'
• B. Mother's group 'A' and father's group 'B'
• C. Mother's group 'O' and father's group 'A'
• D. Mother's group 'B' and father's group 'AB'

Hint:

ABO hemolytic disease of the newborn needs a mother who is blood group O, since only her anti-A and anti-B antibodies are mostly IgG.

Answer 1

The Correct Option is C

Step 1: Recall what ABO incompatibility needs.
ABO hemolytic disease of the newborn happens when the mother's antibody against an A or B antigen crosses the placenta and attacks fetal red cells that carry that antigen. Only an antibody of the IgG type is small enough to cross the placenta in any meaningful amount.

Step 2: Recall which blood group carries that antibody.
People of blood group O naturally carry anti-A and anti-B antibodies that are largely of the IgG type, so these can cross into the fetal circulation. People of blood group A carry anti-B, and people of blood group B carry anti-A, but these antibodies are mostly IgM, which does not cross the placenta in a significant way.

Step 3: Apply this to the combinations given.
For the fetus to be harmed, the mother must both lack an antigen the baby carries and hold an IgG antibody against it. Only a mother who is blood group O fits this, since her anti-A and anti-B are IgG. If the father is group A, the baby may inherit the A antigen that the O mother's antibody can attack.

Step 4: Rule out the other combinations.
A mother who is group A, whether the father is O or B, still keeps her as group A, so her only relevant antibody, anti-B, is mostly IgM and rarely crosses in a harmful amount. A mother who is group B similarly keeps an IgM predominant anti-A. None of these combinations places the mother in the high risk O group.

Step 5: Flag the discrepancy from the source.
The printed answer key attached to this question marks the combination of mother group A and father group B as correct. That does not match the standard immunology of ABO disease, since a group A mother's antibody is mainly IgM. We have corrected this to the combination that fits established teaching, mother group O and father group A, and flag the source key as likely mistaken.

Final Answer:
ABO incompatibility is most common when the mother is blood group O and the father is blood group A.