Platelet immunology. ELISA for detection of platelet antibodies, platelet-specific antigens and platelet glycoproteins.

The development of an enzyme linked immuno-sorbent assay (ELISA) for detection of platelet antibodies and platelet specific-antigens has been described. In the assay, intact platelets fixed to the bottom of microplates have been used as targets. After incubation with serum, the attached antibody has been detected by antiglobulin serum labelled with enzyme followed by assay of the enzyme reaction with its substrate. The clinical significance of platelet antibodies in alloimmune neonatal thrombocytopenia (AINT), post-transfusion purpura (PTP), and refractoriness to platelet transfusion therapy has been described. The clinical significance of autoantibodies in autoimmune thrombocytopenic purpura (AITP) and the detection of platelet surface glycoproteins by a modification of the platelet ELISA has also been described. In addition, a survey of the present state of knowledge of human platelet immunology, especially platelet-specific alloantigens and alloantibodies has been given. The studies of AINT and PTP have revealed a human immune response gene located in the MHC region: The susceptibility to anti-Zwa alloimmunization is strongly associated with the HLA-B8 and even more DR3 and DRw52a tissue types. In the studies of PTP, the first example of PTP due to anti-Zwb (only detectable in the platelet ELISA) has been described, and an association of the presence of anti-Zw antibodies of IgG3 subclasses (defined in the platelet ELISA using monoclonal mouse antibodies) and destruction of autologous platelets has been described. In an investigation of Ig classes and IgG subclasses of platelet associated Ig (PAIg) in children suffering from autoimmune thrombocytopenic purpura, no correlation was found between the immunochemical properties of the PAIg and the clinical course of the disease. However, a correlation between increased amounts of PAIgG3 and very low platelet counts was observed. A modification of the platelet ELISA using monoclonal antibodies against glycoproteins (GP) has been described. Using antibodies against GPIb and GPIIb/IIIa, it has been possible to diagnose Glanzmann's thrombasthenia (GT) and Bernard-Soulier syndrome (BSS). Especially in the diagnosis of thrombocytopenic patients suspected for BSS where platelet aggregation studies has been impossible due to low platelet counts, the detection of decreased amounts of surface GPIb in ELISA has been of great value. In conclusion, the platelet ELISA has been found simple and sensitive, and suitable for routine investigations of AINT, PTP, AITP, and refractoriness to platelet transfusion.