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. HPAs are polymorphisms in platelet membrane glycoproteins that can stimulate production of alloantibodies once exposed to foreign platelets with different HPAs. HPAs can also be targeted by autoantibodies and drug dependent antibodies.
Platelet immunofluorescence test (PIFT) and capture assays, such as the Monoclonal Antibody Immobilisation Platelet Antigen (MAIPA) assay can detect platelet alloantibodies.
Currently, 33 HPAs have been identified on six platelet membrane glycoproteins; GPIIb, GPIIIa, GPIbα, GPIbβ, GPIa, and CD 109. For a current list of HPAs, see the Immuno Polymorphism Database – alloantigen/protein data table.
Twelve of the platelet specific antigens are clustered into six biallelic groups (HPA-1, HPA-2, HPA-3, HPA-4, HPA-5 and HPA-15). The antigens are numbered in order of discovery with the higher frequency antigen labelled ‘a’ and the lower frequency antigen labelled ‘b’. HPAs for which antibodies have been identified against one of the two antigens are labelled as ‘w’ for workshop. The allele frequencies for many racial and ethnic populations have been determined.
HPA alloantibodies are responsible for the following clinical conditions:
- Fetomaternal or neonatal alloimmune thrombocytopenia (FNAIT/NAIT)
- Post transfusion purpura (PTP)
- Platelet transfusion refractoriness
The majority of HPAs are expressed on the GPIIb and GPIIIa glycoprotein complex on the platelet membrane. This complex appears to be very immunogenic. HPA-1a antibodies account for greater than 80% of the HPA specific platelet antibodies detected in sera of alloimmunised people. These antibodies account for the majority of FNAIT cases. HPA-1b antibodies are frequently detected in patients with PTP.
Platelet complex GP11b/IIIa plays a vital role in haemostasis. Patients with Glanzmann thrombasthenia who either lack this complex or it is dysfunctional due to inherited mutations experience serious bleeding. These patients, when exposed to normal platelet transfusions can make antibodies against GP11b/IIIa.
HPA matched platelets can be supplied after consultation with an Australian Red Cross Blood Service Transfusion Medicine Specialist for the following indications:
- Thrombocytopenia due to alloimmunisation ie neonates with FNAIT
- Patients who are refractory to random donor platelet transfusions due to the presence of HPA alloimmunisation
- AABB Technical Manual. 18th ed. USA, 2014.
- Curtis BR, McFarland JG. Human platelet antigens - 2013. Vox Sanguinis 2014:106:93-102.