ABO Incompatibility- Important Concepts.

 

ABO Incompatibility- Important Concepts.            Pradeep Alur, MD.

 

1.  ABO incompatibility exists only when mother is “O” and the baby is A or B blood group. Why?
2. As all those with O blood group will have anti-A and anti-B antibodies in their serum, why do we still do Coombs (Indirect) test for antibodies in the mother?
3. If the mother with O blood group has positive Coombs test, do we still need to do Coombs in the baby?
4. If the mother is Coombs test negative with O blood group, do we still need to do Direct Antibody (Coombs) test in the baby with A or B blood group?
5. Is there relevance in doing Coombs test in both the baby and the mother? Should we restrict it to the baby alone?

 

 

 

 

ANSWER 1-- ABO incompatibility occurs only when a mother has blood type O and the baby has either blood type A or B.

• Explanation: In nature, individuals with blood types A and B produce anti-B and anti-A antibodies in their serum, respectively. These antibodies are of the IgM type, which do not cross the placenta. However, about 50% of people with blood type O have IgG-type anti-A and anti-B antibodies in their serum. Unlike IgM antibodies, IgG antibodies can cross the placenta, potentially leading to hemolysis in the baby.

 

 

 

ANSWER 2- Since all individuals with blood group O possess anti-A and anti-B antibodies in their serum, why is the Coombs (Indirect) test still performed to detect antibodies in the mother?

• The Coombs test is designed to identify IgG-type antibodies against A and B blood groups, which are found in about 50% of individuals with blood group O. Since IgG antibodies have clinical significance, an Indirect Coombs test is conducted on the mother.

 

 

 

 

ANSWER 3- If a mother with O blood group has a positive Coombs test, is it still necessary to perform a Coombs test on the baby?

• Explanation: IgG has four subclasses: IgG1, IgG2, IgG3, and IgG4. The G2 and G4 subclasses do not bind to the antigens on red blood cells (RBCs), while only G1 and G3 subclasses can adhere to these antigens. Since the Coombs test does not differentiate between the subclasses of IgG, it is essential to perform a Coombs test on the baby as well.

 

 

 

ANSWER 4- If a mother is Coombs test-negative and has an O blood group, is it still necessary to perform a Direct Antibody (Coombs) test on her baby if the baby has an A or B blood group?

• Explanation: Studies have demonstrated that babies who test positive on the Coombs test often come from mothers who have only a one-third chance of also testing positive. This is due to the fact that the direct Coombs test is more sensitive than the indirect Coombs test. Therefore, it is advisable to conduct a direct Coombs test on the babies, even if the mother’s Coombs test result is negative.

 

 

 

ANSWER 5-  Is there relevance in performing the Coombs test on both the baby and the mother, or should it be restricted to the baby alone?

The studies show that when the Coombs test is positive for both the mother and the baby, the risk of severe jaundice requiring phototherapy is high.