Residual MHC class II positive B cells are responsible for the reduced platelet immunity. BALB/c MC were depleted (-) of either T cells (second column), NK cells (third column), B cells (fourth column), or MHC class II⁺ cells (fifth column) by MACS, added to platelets (at 1/μL) from anti-AsGm1–treated SCID mice, and transfused weekly into CBA recipients. The data is expressed as the mean ± SD of the week 8 titers from 10 recipient mice. The first column represents control transfusions with anti-AsGm1–treated SCID mouse platelets alone.