You've likely heard your blood type mentioned, perhaps as 'A positive' or 'O negative.' But what does the 'RhD negative' part really signify? It's a fascinating aspect of our unique biological makeup, determined by our genes and inherited from our parents.
Essentially, if you're RhD negative, it means your red blood cells don't have a specific protein on their surface called the Rhesus D antigen. In the UK, about 15% of people fall into this category, while the majority, around 85%, are RhD positive. This difference might seem small, but it can become quite significant, particularly during pregnancy.
Why does this matter when you're expecting? The key concern arises if an RhD negative mother carries an RhD positive baby. This situation, often referred to as 'incompatibility,' can only occur if the baby's biological father or sperm donor is RhD positive. It's not a guarantee, though, as the father might carry genes for both positive and negative types. If you're RhD positive and your baby is RhD negative, there's generally no cause for concern regarding this specific aspect.
During pregnancy, there's a possibility that a small amount of the baby's blood cells could enter the mother's bloodstream. This event is called a feto-maternal haemorrhage (FMH). If the baby is RhD positive and their blood cells reach the RhD negative mother, her immune system might perceive the RhD antigen as a foreign substance. This can trigger the production of antibodies against it, a process known as sensitisation or alloimmunisation. The likelihood and severity depend on the volume of blood exchanged and the mother's immune response.
While FMH can happen at various points during pregnancy – including miscarriages, abortions, or even after minor injuries or bleeding – the most common time for it to occur is during childbirth. Any event that could lead to the mother producing antibodies against the D antigen is termed a 'potentially sensitising event.'
Once sensitisation happens, it's permanent. The antibodies created then reside in the mother's blood. If she becomes pregnant again with an RhD positive baby, these antibodies can cross the placenta and attack the baby's red blood cells. This can lead to a condition called haemolytic disease of the fetus and newborn (HDFN), sometimes called 'Rhesus disease.'
HDFN can range from mild, causing jaundice that requires a longer hospital stay for the baby and possibly a transfusion, to very severe. In severe cases, it can sadly result in stillbirth, death after birth, or permanent disabilities for the baby. Because the antibodies remain, HDFN can affect subsequent pregnancies too.
Fortunately, HDFN is now uncommon thanks to a highly effective preventative measure: anti-D prophylaxis. This involves administering a special injection of anti-D antibodies to RhD negative mothers. Given around 28 weeks of pregnancy and again after birth, this injection significantly reduces the chance of sensitisation to a very low level (around 0.35%). Anti-D is also recommended as a treatment if a potentially sensitising event occurs during pregnancy. It's crucial to remember that without this preventative treatment, sensitisation can occur, and once it has, anti-D is no longer effective. Sensitisation is irreversible.
So, what exactly is anti-D prophylaxis? It's a medicine designed to prevent a harmful condition from developing. By giving you these ready-made antibodies, it stops your body from producing its own antibodies against RhD positive blood cells, thereby preventing sensitisation and the subsequent risk of HDFN. These antibodies are carefully collected from plasma donors with high levels of anti-D and are processed under strict controls to ensure safety. As with any medical treatment, there's a very small risk of allergic reaction, which is why a short waiting period after administration is often recommended.
