Antigenic determinants in idiopathic thrombocytopenic purpura.

Serum platelet bindable immunoglobulin (SPBIg) was determined in a group of 23 idiopathic thrombocytopenic purpura (ITP) patients and compared to 20 normal, healthy controls. The mean SPBIg of the ITP group was 16.1 (+/- 17.9 SD) fg/platelet, while the normals were substantially lower, 4.0 (+/- 1.2) fg/platelet. Sera from patients of both groups were then incubated with platelet fractions immobilized on nitrocellulose membrane strips (Western Blotting) to detect platelet antigen specificity using a peroxidase labelled indicator antibody. The normal patient sera did not react with platelet fractions on the nitrocellulose strips. However, 21 of 23 ITP sera bound to one or more platelet fractions with large variations in the number and molecular weights of the platelet fractions identified by ITP antibody. These observations suggest the presence of multiple antigenic binding sites for platelet specific immunoglobulin in ITP sera. This variation may reflect heterogeneous antibodies binding to diverse antigens or homogeneous antibodies to a limited number of antigenic determinants shared by several discrete platelet molecules.