Anti-β3 integrin antibodies were generated in β3^−/−FcRn^−/− mice after immunization with β3^+/+FcRn^−/− platelets. (A) β3^+/+FcRn^−/− mouse platelets were incubated with a 1:100 dilution of sera from β3^−/−FcRn^+/+ mice immunized either 2 (gray) or 4 (black) times weekly or preimmune sera (filled area) and stained with FITC-conjugated goat anti–mouse IgG or rat anti–mouse IgG₁, IgG_2a, IgG_2b, or IgG₃. (B) Thrombocytopenia was observed in β3^+/+FcRn^−/− mice after intraperitoneal injection of 25 μL of antisera from β3^−/−FcRn^−/− mice transfused either 2 times (▴) or 4 times (▾) with β3^+/+FcRn^−/− platelets. PBS (■) was used as a control. (C) 50 μL of mouse anti-β3 integrin sera (50 μL; from 4× transfused β3^−/−FcRn^−/− mice) or (D) IVIG (10 μL; 10%) were injected intraperitoneally into 6- to 8-week-old β3^−/−FcRn^−/− or β3^−/−FcRn^+/+ littermate mice. The concentration of retained IgG was determined. Adult β3^−/−FcRn^−/− mice cleared IgG rapidly compared with β3^−/−FcRn^+/+ mice. n = 3-5 in each group.