Alloantigen presentation and recognition in MHC-matched and MHC-mismatched transplantation. Peptide recognition in MHC-matched transplant (left). Because donor T cells are selected through positive and negative selection in donor thymus, the donor TCR recognizes host-derived (non-self) mHAs (green) within an MHC (red) that is shared between donor and host. Molecular mimicry in MHC-mismatched transplantation (right). The donor TCR can recognize a mismatched host MHC (blue) loaded predominantly with antigenic peptide derived from ubiquitous self-proteins (yellow) rather than polymorphic mHAs.