Allogeneic platelet MHC class I antigens rescue megakaryocyte abnormalities in T-cell–mediated ITP. Representative hematoxylin and eosin stained bone marrows of SCID mice transferred with CD19-depleted splenocytes from (A) a control BALB/c naïve mouse or BALB/c CD61 KO mice immunized against (B) BALB/c (CD61⁺/H-2^d) platelets; (C) C57BL/6 (CD61⁺/H-2^b) platelets; or (D) C57BL/6 CD61 KO (CD61⁻/H-2^b) platelets. The bars in each panel represent 50 μm. The yellow arrows point to megakaryocytes. (E) Cumulative results of megakaryocyte enumeration in the bone marrow of control naïve SCID mice (hatched column), SCID mice transferred with either nondepleted splenocytes (antibody-mediated ITP) from the indicated platelet donors (white columns), and SCID mice transferred with CD19-depleted splenocytes (T-cell–mediated ITP) from the indicated platelet donors (black columns). At least 10 fields in each bone marrow were counted and the results are presented as the mean megakaryocyte number per field (***P < .001). Images of all panels were taken at ×40 original magnification with a 0.75 numerical aperture of the objective lens.