ABO Incompatibility

How ABO blood groups work

Red blood cells carry A antigens, B antigens, both (AB) or neither (O). From early childhood, people naturally make antibodies against the antigens they lack:

If cells meet antibodies against them, the antibodies attach to the cells and destroy them. This is called hemolysis, the same process seen in hemolytic anemia.

ABO incompatibility in newborns

Why it happens

People with type O blood often make a form of anti-A and anti-B antibodies (IgG) that can cross the placenta. If a type O mother carries a baby with type A or B blood, some antibodies reach the baby and attack the baby's red cells. Unlike Rh disease, ABO incompatibility can affect a first pregnancy, and it does not usually get worse in later pregnancies.

How common is it

ABO mismatch with a type O mother and type A or B baby occurs in roughly 15% of pregnancies, but only a small fraction of these babies develop significant hemolysis. The disease is usually milder than Rh disease because A and B antigens are less developed on fetal red cells and are also found on other tissues, which soak up some antibody.

Symptoms in the baby

• Jaundice (yellow skin and eyes), often within the first 24 hours, earlier and faster rising than normal newborn jaundice
• Mild anemia, sometimes with pale skin
• Occasionally an enlarged liver or spleen

Severe hemolysis before birth, causing swelling of the baby (hydrops), is very rare in ABO disease.

Diagnosis

Babies with early or significant jaundice have blood tests:

• baby's blood group and the mother's
• direct antiglobulin test (DAT, or Coombs test): shows antibodies coating the baby's red cells, though it can be negative in ABO disease
• bilirubin level, measured in blood or through the skin
• hemoglobin and reticulocyte count (young red cells), and a blood smear showing spherocytes (round red cells)

Treatment

• Phototherapy: blue light breaks down bilirubin in the skin. It is the main treatment and is very effective.
• Feeding support: frequent feeding helps bilirubin leave the body in stool.
• IV immunoglobulin (IVIG): used when bilirubin keeps rising despite phototherapy, to reduce hemolysis.
• Exchange transfusion: rarely needed, for very high bilirubin levels, to remove bilirubin and antibodies.
• Follow-up blood count: some babies develop late anemia at 2 to 6 weeks and occasionally need a transfusion.

Complications

Very high bilirubin can enter the brain and cause kernicterus, with permanent brain damage, hearing loss and movement problems. This is preventable with timely testing and treatment. Babies with ABO incompatibility who have a positive DAT are considered at higher risk under jaundice guidelines and are monitored more closely.

ABO incompatibility in blood transfusions

Giving blood of an incompatible ABO group causes an acute hemolytic transfusion reaction. The recipient's antibodies rapidly destroy the donor cells. Most cases result from identification errors, such as a sample from the wrong patient or the wrong unit given.

Symptoms can start within minutes:

• fever and chills
• back or flank pain, chest tightness
• breathlessness, low blood pressure, fast heart rate
• red or dark urine
• bleeding from abnormal clotting
• kidney failure in severe cases

The transfusion must be stopped immediately and emergency treatment given. Strict checks of patient identity, blood group and crossmatching at the bedside make these reactions rare. ABO incompatibility also matters in organ transplantation, though some kidney transplants can now be done across blood groups with special preparation.

Prevention

• Blood group testing of all pregnant people early in pregnancy.
• Close monitoring of jaundice in newborns, particularly those of type O mothers, with bilirubin checks before discharge and follow-up within a few days.
• Careful patient identification and compatibility testing before every transfusion.

There is no injection to prevent ABO disease, unlike the anti-D injection used for Rh disease.

Frequently asked questions

What is ABO incompatibility in a newborn?

It happens when a mother with type O blood has a baby with type A or B blood. The mother's antibodies cross the placenta and break down some of the baby's red cells, causing jaundice and sometimes mild anemia in the first days of life.

Is ABO incompatibility serious?

In most newborns it is mild and easily treated with phototherapy. A small number need more treatment. Untreated, very high bilirubin can cause brain damage, which is why jaundiced babies are monitored closely. ABO incompatible transfusions can be life-threatening but are rare.

Can ABO incompatibility affect the first baby?

Yes. Unlike Rh disease, ABO incompatibility can affect the first pregnancy, because type O mothers already have anti-A and anti-B antibodies before pregnancy. It does not usually become more severe in later pregnancies.

If I am O positive and my baby is A positive, will my baby have problems?

Most babies in this situation are healthy or have only mild jaundice. Your baby's jaundice will be checked after birth, and treatment with phototherapy is straightforward if bilirubin rises. Tell your baby's doctor if jaundice appears in the first 24 hours.