Human platelet alloantigens (HPAs)

To date, 35 human platelet alloantigens (HPAs) have been identified on seven functionally significant platelet glycoprotein (GP) complexes, which are associated with alloimmune platelet disorders including fetal and neonatal alloimmune thrombocytopenia (FNAIT), post-transfusion purpura (PTP), and multiple platelet transfusion refractoriness (MPR). Among these recognized HPAs, approximately 20 (out of 35) are located on the GPIIb/IIIa complex, a crucial receptor involved in hemostasis and inflammation. Of the recognized HPAs, 12 are present as six serologically and genetically defined bi-allelic systems, where the -a form represents higher frequency alleles and the -b form represents lower frequency alleles. The remaining 21 HPAs are low-frequency or rare antigens, with the -a allele frequency not yet fully determined for antibody specificity.

CD36

CD36 is an 88 kDa transmembrane glycoprotein expressed in various cell types, including platelets, monocytes, macrophages, and endothelial cells. It functions as a multifunctional receptor involved in coagulation, immune response, and fatty acid metabolism. CD36 deficiency is classified into two types: Type I (absence of CD36 on both platelets and monocytes) and Type II (CD36 deficient only on platelets). Individuals with Type I deficiency are generally healthy but may develop anti-CD36 antibodies, leading to bleeding disorders such as platelet transfusion refractoriness (PTR) and fetal neonatal alloimmune thrombocytopenia (FNAIT). The CD36 gene, located on chromosome 7, contains 15 exons, 12 of which are coding, and is highly polymorphic, with multiple mutations linked to deficiency.