MC affect platelet-induced IgG antidonor isotype production. CBA recipient mice (n = 10) were transfused weekly with either (A) control transfusions with 10⁵ BALB/c MC, (B) 10⁸ BALB/c mouse platelets, or (C) 10⁸platelets from SCID mice and their sera tested for the presence of IgG1 and IgG2a antidonor antibodies. The data are representative flow cytometric histogram analyses of IgG1 and IgG2a antidonor isotypes in a recipient’s serum (1/25) at week 8 transfusion; all mice gave similar results. In all panels, prebleed IgG antidonor (), IgG1 (- - -), and IgG2a () reactivity.