A sick baby was successfully transfused with blood from the father who has blood group AB. What is the group of the the sick baby?

Analyze the question: A baby was successfully given blood from a father with blood group AB. We need to find the baby's blood group. "Successfully" means there was no agglutination (clumping) of red blood cells.

Concept: Blood groups are defined by the antigens on red blood cells and the antibodies in plasma.

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

Solution:

1. The donor (father) is AB, so his red cells carry both antigen A and antigen B.
2. For the transfusion to succeed, the baby's plasma must not contain anti-A or anti-B antibodies, otherwise the donor's cells would clump.
3. Check each option: • A: has anti-B, which attacks the donor's B antigen, so it is incompatible. • B: has anti-A, which attacks the donor's A antigen, so it is incompatible. • AB: has no antibodies, so it is compatible. • O: has both anti-A and anti-B, so it is incompatible.
4. Only an AB recipient can safely receive AB blood (in standard ABO matching).

Answer: The baby's group is AB, which is option C.

Tip: Remember that AB is the universal recipient (it can receive from A, B, AB and O) and O is the universal donor. Strictly, AB recipients can receive any group, but AB donors can give only to AB. So when the donor is AB and the transfusion worked, the recipient must be AB.