Fetal and neonatal alloimmune thrombocytopenia (FNAIT) results from a mismatch between the human platelet antigen (HPA) of the mother and fetus, causing the mother to produce antibodies that attack the baby’s platelets. HPA-1a antibodies are usually considered the most common cause of FNAIT, followed by HPA-5b antibodies.
Disease presentation can vary widely depending on which HPA type is implicated, so knowing which HPA antibodies you have can help guide your treatment decisions.
Prevalence
The distribution of HPA antibodies causing FNAIT differs across populations. Among white individuals, for example, HPA-1a antibodies account for 80% to 90% of all FNAIT cases. In people with African and Japanese ancestry, on the other hand, HPA-5 antibodies are more common, though further studies are needed to determine their exact prevalence.
Read more about FNAIT causes and risk factors
Some research has shown that the antigens that most often cause FNAIT, primarily HPA-1 and HPA-5 antigens, have somewhat similar frequencies across diverse populations. Although most studies to date have been conducted in Caucasian populations, these findings suggest that the ethnic distribution of FNAIT cases may actually be greater than we currently believe.
Disease severity
There has been some debate over whether HPA-5b antibodies are a true cause of FNAIT or are only an incidental finding. This is because in general, children of mothers with HPA-5b antibodies appear much less likely to experience severe complications of FNAIT than children of mothers with HPA-1a antibodies.
Because HPA-5b antibodies are relatively unlikely to cause severe disease, many experts also recommend that it shouldn’t be included in screening panels for FNAIT. The reasoning behind this is that if included, many low-risk patients might end up receiving unnecessary treatment.
This doesn’t necessarily mean that HPA-5b antibodies can’t cause FNAIT. Rather, the existing evidence simply suggests that severe thrombocytopenia and bleeding complications seem unlikely among these patients.
This is quite different from HPA-1a antibodies, which are a known cause of FNAIT and can result in brain bleeds (also known as intracranial hemorrhage) and severe thrombocytopenia. However, some infants with HPA-1a FNAIT can also experience mild disease or even be asymptomatic.
Treatment options
Several treatment options are available for individuals impacted by HPA-1a FNAIT, including intravenous immunoglobulin (IVIG) therapy, platelet transfusions or corticosteroids, all of which can be given either during pregnancy or after birth.
With HPA-5b FNAIT, the treatment course is less certain. Because cases are usually mild, IVIG therapy usually isn’t recommended, as the risks may outweigh the benefits.
As more studies and clinical trials include patients with both HPA-1a and HPA-5b incompatibility, we will be able to improve our understanding of which treatment methods work best for each group.
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