Alloantibody, autoantibody, and platelet responses observed in all immunizations. Identity of strains used as platelet donors and recipients and number of mice in each of the 8 groups are shown in Table 2. (A) Alloantibody responses were strongest in immunizations involving at least 1 wild-derived mouse strain (PWK, SPRET, groups 3-8). Much weaker alloresponses were seen in reciprocal immunizations involving MHC-matched C57 and 129 (group 2) that differed by only 2 aa in GPIIb. No significant responses were seen following immunization with strain-identical platelets (group 1). *P < .05; **P < .01; ***P < .001. (B) Autoantibody responses were identified in 33 of 48 mice injected with platelets from other strains (groups 2-8). Using the mean + 3 standard deviation (1.8) obtained with the strain-identical immunizations as the cutoff for a "positive" reaction. (C) Platelet nadirs (percentage of maximum platelet count) in mice of groups 1 through 8. (D) There was an inverse correlation between platelet autoantibody levels and platelet nadir (Spearman coefficient P < .0001; r = −0.75). NS, not significant.