Red Blood Cell Alloimmunisation

Alloimmunisation in Pregnant Women

In some cases, a pregnant mother can produce antibodies that can cross the placenta to her unborn baby, a process called alloimmunisation. These antibodies can destroy her unborn baby’s blood cells, leading to jaundice or serious complications known as haemolytic disease of the newborn. Alloimmunisation usually happens when a mother is Rhesus negative and her baby is Rhesus positive. A simple injection can help to stop the process of alloimmunisation.

The Immune System and Alloimmunisation 

The way our immune system works is that it naturally recognises anything ‘foreign’ that could do us harm. Our immune system then produces antibodies to fight the foreign substance it has identified. For example, our immune system recognises germs in our bloodstream and produces antibodies to fight those germs. By destroying the germs, we are protected from infections that make us unwell. When the immune system has been triggered, it remembers the foreign invader and produces the antibodies every time it recognises that particular germ. 

The immune system can also view cells from a different person as ‘foreign’ and produce antibodies to destroy them. This can happen with a blood transfusion if the blood you receive is a different blood group to yours. The process is called alloimmunisation. In the case of blood transfusions, it can be prevented by ensuring the blood transfused to a woman is carefully matched to her own blood type. 

Alloimmunisaition can also happen if a baby’s blood enters the mother’s circulation, so it is important that this is picked up in pregnancy. If it is not, the baby is at risk for haemolytic disease.

Haemolytic Disease

If a baby has a different blood group to their mother’s and the mother produces antibodies that cross the placenta to the baby’s blood cells, a baby can develop haemolytic disease. If a baby has this condition, their red blood cells are destroyed faster than their body can replace them. 

Haemolytic disease can result in serious problems, including a baby suffering from severe anaemia and heart failure, either before or after he or she is born. The infant might need intensive care, blood transfusions and other treatments. In some very serious cases, babies can die before they are born, or soon after they are delivered. 

The mother’s future pregnancies will also be at risk because her immune system will retain the antibodies for many years. 

How Common Is Alloimmunisation in Pregnancy?

Only about one in a 100 pregnant women will actually be affected by alloimunisation and need special care during pregnancy. 

Approximately 85 percent of mothers with red blood cell alloimmunisation in pregnancy have a Rhesus blood group. The mother may produce antibodies if she is Rhesus negative and her baby is Rhesus positive. Other red blood cell groups can be a problem, but they are rare.

A woman can become alloimmunised because of:

  • A previous blood transfusion. 
  • A previous pregnancy. 
  • Past miscarriages or abortions. 

A previous pregnancy is the most common reason for alloimmunisation. This is because the mother produces the antibodies in response to the baby’s blood crossing the placenta into her bloodstream during the birth. During a first pregnancy, the baby is usually born before the mother’s immune system has produced many antibodies. 

Therefore, if a woman with Rhesus negative blood has a baby with Rhesus positive blood, her body will produce the antibodies that could cause her next Rhesus positive baby to become unwell with haemolytic disease. 

The Anti-D Injection  

Treating a mother with a Rhesus blood group involves an anti-red blood cell antibody called the Rhesus ‘anti-D’. Anti-D immunoglobulin does not stop the baby’s red blood cells from entering the mother’s circulation, but it does stop the mother’s immune response from triggering the antibodies that could harm her baby. It works by removing the foetal blood cells from the mother’s blood before she becomes sensitised to these blood cells and produces the antibodies that could harm her unborn baby. 

Anti-D immunoglobulin is collected from the blood of healthy adults and is tested to ensure that it is safe to use on pregnant mothers. It is given to the pregnant mother by way of injection at various stages of her pregnancy and at birth, if required. 

Pregnant Women at Risk from Alloimmunisation

Alloimmunisation is more likely to occur in pregnant women with vaginal bleeding, or if they suffer an injury to their abdomen. Mothers who undergo an amniocentesis are also more at risk since this procedure involves inserting a needle into the womb. This could result in the baby’s blood entering the mother’s circulation, which could trigger the antibody production called alloimmunisation. 

If a pregnancy is breech and the doctor or midwife attempts to turn the baby in the mother’s womb, this can also produce a risk of alloimmunisation. A mother who is Rhesus negative will need additional anti-D injections if any of these situations apply during her pregnancy. You must,  therefore, let your doctor or midwife know immediately if any of these situations apply to you. 

Diagnosing and Preventing Alloimmunisation

During your first antenatal appointment, you will have a blood test to identify your blood group. This blood test will also detect the presence of anti-red blood cell antibodies.

  • The test will show the presence of anti-red blood cell antibodies in about one percent of pregnant women.
  • If your doctor or midwife identifies antibodies, they will need to establish the baby’s blood group so they can determine whether the baby is at risk of anaemia. 
  • The only certain way to determine a baby’s blood group is with an amniocentesis, which is performed at 15 weeks of gestation. During this procedure, a small amount of fluid is taken from around the baby using a needle. 
  • The father of the baby may also need a blood test to help your doctor determine your baby’s blood group. 
  • Approximately 15 percent of women need treatment to prevent alloimmunisation from occurring when they are pregnant and during childbirth. 

Pregnant Women with Red Blood Cell Alloimmunisation 

Anti-D injections prevent a mother from forming antibodies to red blood cells. If antibodies have already been produced, there is no benefit to be gained from having anti-D injections. 

The higher the amount of antibodies, the higher the risk to the unborn baby. 

If your doctor determines that you are medium or high-risk, you may need regular blood tests to measure the level of antibodies you are producing. You may also need ultrasound examinations to look for signs that your baby is anaemic. 

If your baby is at risk of haemolytic disease, your doctor will advise you what to do. Your baby might need fetal therapy, early delivery, or ultra-sound guided blood sampling. A baby that is very premature and severely anaemic might need a blood transfusion, which is performed by a highly-trained specialist.

Rhesus Negative Mothers 

If you are Rhesus negative but no anti-red blood cell antibodies are detected at your antenatal visit, you will be asked to attend for a further blood test at 28 weeks. You will be tested again when your baby is due to be delivered.  

All Rhesus negative women should be offered an injection of anti-D at 28 weeks. When your baby is born, your doctor or midwife will test your baby’s blood group using blood from the umbilical cord. If your baby’s blood group is Rhesus positive, you will need another dose of anti-D.