Sera from HIV-1-ITP patients contain IgG Abs against platelets and an immunodominant GPIIIa-49-66 peptide as measured by ELISA. Serial dilutions of sera from HIV-1-ITP patients (D, L, and F) were tested for binding to platelets immobilized on MCP microtiter plates (A) and to a GPIIIa-49-66 peptide immobilized on ELISA plates (B). (A) In the antiplatelet assay, a high-titer serum to the platelet alloantigen HPA-1a was used as a positive control, whereas a normal serum containing no platelet Abs was used to measure background level. (B) In the GPIIIa-49-66 peptide ELISA, serum from 3 HIV-1 patients without HIV-1-ITP (1-3) and one healthy donor (Normal) were included. The data presented in panels A and B are the median of 2 separate experiments. A₄₀₅ indicates an optical density of 405 nm.