The placenta plays a protective role the fetus during pregnancy. It acts as a buffer–a physical barrier between the maternal and fetal blood circulation.
The placenta typically can prevent maternal antibodies and immune cells from attacking the fetus, as it selectively allows the transfer of antibodies that can function as passive immunity for the fetus.
However, in fetal and neonatal alloimmune thrombocytopenia (FNAIT), maternal antibodies do pass through the placenta and attack fetal platelets, potentially resulting in bleeding complications that may even be fatal. Here’s some more information to help you understand why this happens.
When the placenta is not enough
In FNAIT, the normally protective role of the placenta is insufficient to prevent the crossing over of damaging maternal antibodies to the fetus. There are a few factors that contribute to this.
First, in FNAIT, the mother’s body produces high-affinity antibodies against fetal platelets antigens. In other words, these maternal antibodies have special properties that cause them to bind strongly to neonatal Fc receptors, which act as gatekeepers in what is allowed to pass through the placenta.
Read more about FNAIT testing and diagnosis
Second, it is important to consider how maternal antibodies against platelets are produced in FNAIT. These antibodies are produced following what is known as a “sensitizing” event. This event can occur during a previous pregnancy/miscarriage with a fetus with human platelet antigen (HPA) incompatibility; this encounter causes the maternal immune system to produce the antibodies responsible for FNAIT, as well as memory B cells. Once these are in the maternal circulation, the placenta is unable to neutralize them.
Third, the placenta doesn’t have a repair mechanism once these antibodies cross over to attack the fetal platelets. Once these antibodies reach the fetus, platelet destruction occurs in the spleen and liver of the fetus. The destruction of platelets means that the blood cannot clot normally when an injury occurs. This means that bleeding in the brain—one of the most serious complications of FNAIT— may occur even before birth.
Staying safe during pregnancy
To avoid FNAIT complications in a present or future pregnancy, it is important to let your doctor know if you have a previous history or suspected history of FNAIT. This will allow your doctor to initiate surveillance and the necessary therapies to prevent complications from occurring.
In FNAIT, it is not the placenta that fails; rather, it is that the antibodies have found a way to bypass the normal checks to cross through the placenta to cause harm to the fetus. With adequate antenatal management, the risk of injury to your fetus can be significantly minimized.
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