Immune mechanisms in platelet clearance induced by prior transfusion. Alloantibodies against MHC antigens made by B cells can develop in patients after blood transfusion and are associated with decreased survival of transfused platelets. The clearance of platelet-alloantibody complexes by the reticular endothelial system has long thought to be disease causative in patients refractory to platelet transfusion. CD4⁺ T cells interact with donor MHC molecules on APCs, and once activated, provide help to B cells to drive the production of alloantibodies. In the accompanying study, removing B cells (and correspondingly anti-MHC alloantibodies) had little effect on the poor long-term platelet survival in alloimmunized mice. However, the depletion of CD8⁺ T cells significantly improved the survival of transfused platelets. Platelets are depicted in red; (allo) antibodies in yellow; MHC antigen on APC in blue; CD4 T-cell receptor in purple, and CD8 T-cell receptor in green. These findings support a direct role for T cells in platelet clearance induced by prior blood transfusion. Professional illustration by Somersault18:24.