Human platelet antigens

Platelets express both platelet specific antigens such as human platelet antigens (HPAs) and antigens which are shared with other cells such as ABO and HLA class I antigens.

Currently, 35 HPA systems have been identified on six platelet membrane glycoproteins and of these, HPA-1 to HPA-5 and HPA-15 are biallelic, resulting in a total of 41 antigens.

HPAs are polymorphisms in platelet membrane glycoproteins that can stimulate the production of alloantibodies after exposure to platelets with different HPA specificity through transfusion or pregnancy. HPAs can also be targeted by autoantibodies and drug-dependent antibodies.

HPA antibodies can be implicated in the following clinical conditions:

• Fetomaternal or neonatal alloimmune thrombocytopenia (FNAIT/NAIT)
• Post transfusion purpura (PTP)
• Platelet transfusion refractoriness

HPA-1a antibodies account for greater than 80% of the HPA antibodies detected and are the most common cause of both FNAIT and PTP.

HPA-matched platelets can be supplied after consultation with a Lifeblood Transfusion Medicine Specialist for the following indications:

• Thrombocytopenia due to alloimmunisation i.e. neonates with FNAIT.
• Patients who are refractory to random donor platelet transfusions due to the presence of HPA alloimmunisation.

HPA antibodies can be detected using the platelet immunofluorescence test (PIFT) and capture assays, such as the monoclonal antibody immobilisation platelet antigen (MAIPA).

Updated March 2026