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What causes FNAIT ?

HPA-1a incompatibility

HPA-1a is a protein (antigen) which 98% of Caucasians have on the surface of their platelets. The problem is that 2% of pregnant Caucasian women do not have this antigen (they are said to be HPA-1a negative). Nearly all women in other ethnic groups are HPA-1a positive which reduces the risk of HPA-1a incompatibility in these other populations.


If an HPA-1a negative woman gets pregnant, her fetus will most often be HPA-1a positive because it inherits the antigen from the father. Such a pregnancy is said to be incompatible for the HPA-1a antigen. Fortunately, in most pregnancies this does not matter because the blood of the baby and the blood of the mother are kept completely separate.


Immunization of the mother

However, a fetus’ blood may by accident leak into its mother’s bloodstream. This may happen during the pregnancy but in 3 out of 4 cases it happens in conjunction with the birth. If the mother is HPA-1a negative and the fetus HPA-1a positive, the fetal blood will carry fetal platelets with the HPA-1a antigen into the mothers blood stream. Here these platelets may be detected by the mother’s immune system which will then make antibodies against the HPA-1a antigen.


Immune attack on the fetal platelets

The HPA-1a antibodies in an immunized, pregnant woman are able to migrate from the mother’s blood and into the fetus where they will attack and to varying degree cause destruction of the fetus’ platelets if the fetus is HPA-1a positive. If the bleeding and immunization of the mother happen during the same pregnancy, the mother may attack both this fetus and subsequent HPA-1a positive fetuses’ platelets. If, however, the bleeding happens in conjunction with birth, the fetus that caused the immunization is already born and will not be affected by the mother’s HPA-1a antibodies; only subsequent HPA-1a positive fetuses will be attacked.



Today, pregnant women are not tested to see if they are HPA-1a positive (98%) or negative (2%), i.e. if they are at risk of causing FNAIT in their fetus or not. 0.5% are at high risk of causing FNAIT. The lack of diagnosis means that the first baby suffering from FNAIT either dies during the pregnancy or is born with FNAIT-related symptoms ranging from bruises that automatically disappear after birth to massive intracranial bleeding and chronic brain damage.


We at Prophylix fight to change this. We believe that all pregnant women deserve to be tested. General HPA-1a testing will enable specialized physicians identify and monitor at-risk pregnancies an use best medical practice to minimize damages to the fetuses. Furthermore, once Prophylix’s prophylaxes are approved, testing will tell us who to treat and we believe that our prophylaxes will have the potential to essentially eliminate future cases of FNAIT.


Why should we aspire for less?