Binding of human IgG Fab's to purified platelets. Fab supernatants were tested for binding to purified platelets immobilized on (A) MCP microtiter wells or (B) PVDF membranes. BSA was included in the ELISA to evaluate for nonspecific binding. A high-titer platelet alloantigen HPA-1a serum was used as a positive control, whereas a serum containing no platelet Abs and the anti-HIV-1 gp120 Fab 12.9 were used as negative controls. The data presented are the median of 2 separate experiments. Images were scanned using Adobe Photoshop.