Allogeneic platelet MHC class I antigens inhibit T-cell–mediated ITP. Platelet counts in SCID mice transferred with either (A) nondepleted, or (B) CD19-depleted splenocytes from either naïve BALB/c mice (●; N = 18), BALB/c CD61 KO mice immunized against platelets from BALB/c (CD61⁺/H-2^d) mice (○; N = 11), C57BL/6 (CD61⁺/H-2^b) mice (△; N = 18), or C57BL/6 (CD61⁻/H-2^b) CD61 KO mice (□; N = 8). Results are presented as weekly platelet counts (× 10⁹/L) after splenocyte transfer. The dotted horizontal line represents the mouse platelet cutoff counts for thrombocytopenia. In (A) *P < .05 for □ vs △ or ○, and (B) *P < .05 for △ vs ○.