Neonatal Alloimmune Thrombocytopaenia (NAIT)

Neonatal alloimmune thrombocytopaenia (NAIT) results from maternal alloimmunisation against foetal platelet antigens inherited from the father, which are different from those present in the mother. This results in the destruction of foetal platelets by maternal IgG antibodies and presents as a severe isolated thrombocytopaenia in otherwise healthy newborns. Presence of the HLA type HLA-DRB3*01 in the mother has been implicated as the antigen presenting molecule. Approximately 20% of newborn NAIT cases present with intracranial haemorrhage which can be fatal if not treated promptly. Mothers are typically healthy with no previous history of thrombocytopaenia themselves or autoimmune disorders.

 

NAIT is caused by anti-HPA antibodies. Although platelets express HLA antigens, prospective studies have not implicated HLA antibodies in thrombocytopaenia in the absence of neutropeania. Testing therefore involves screening of the mother for circulating anti-HPA antibodies as well as HPA typing of both parents to identify target antigens present in the father which are absent in the mother. HPA antibody testing techniques most commonly used include the flowcytometric Platelet immunofluorescence Test (PIFT) and the Monoclonal Antibody specific Immobilisation of Platelet Antigens (MAIPA) test which uses platelet antigen specific monoclonal antibodies to specifically capture platelet antigens and is therefore useful for differentiating HPA from HLA antibodies. HPA typing is by DNA based techniques. Where the father is heterozygous for the implicated antigen or where paternity is uncertain, the child may need to be HPA typed to confirm a diagnosis of NAIT.

 

Infants with NAIT are at risk of haemorrhage throughout their thrombocytopaenic period, including intracranial bleeding and must be transfused with platelets even before laboratory results are available to confirm the diagnosis. Transfusion before laboratory results are available is with platelets that are HPA-1a5b neg. as these are the two main HPA antibodies implicated in NAIT. 

 

For ongoing support, HPA typing of donors is required to identify donors lacking the antigen against which maternal antibodies have been formed. Mothers with previous NAIT babies need to be closely monitored in subsequent pregnancies.

 

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