Generation of antiplatelet IgG antibodies in CD61 KO mice. Representative flow cytometric histograms of anti-CD61 and/or anti-MHC class I serum reactivity from immunized BALB/c CD61 KO mice. BALB/c CD61 KO mice were transfused weekly (×4) with platelets from either (A) BALB/c (CD61⁺/ H-2^d) mice; (B) C57BL/6 (CD61⁺/ H-2^b) mice; or (C) C57BL/6 CD61 KO (CD61⁻/ H-2^b) mice. Serum was prepared and incubated (1:400 dilution shown) with either BALB/c platelets (top panels), C57BL/6 platelets (middle panels), or C57BL/6 CD61 KO platelets (lower panels), and then incubated with a fluorescein isothiocyanate-conjugated goat anti-mouse IgG secondary antibody, and fluorescence was analyzed by flow cytometry. The thin lines in each histogram indicate the pretreatment serum reactivity, while the thick lines show serum reactivity after 4 platelet transfusions. Splenocytes from the immune mice were then used in the indicated experiments.