Thank you for the presentation. I would like to add some points for the Direct Allorecognition: The MHC molecule of the donor is not processed by the recipient APCs and presented to the recipient T cells, that happens in the indirect way. In the direct allorecognition, MHC molecules of the donor APCs present their own antigens to the recipient T cells. Normally, our T cells are trained to recognize only self-MHC molecules, so actually it is a bit odd that in this case they interact with MHC-Antigen complexes from another individual (which is called allorecognition); but they do. A likely explanation could be that the T cell receptors have some intrinsic affinity for MHC molecules, self or foreign. So, it is an example of an cross-reaction in which a T cell that was selected to be self MHC restricted is able to bind structurally similar allogeneic MHC molecules with high enough affinity to permit activation of T cells. (Abbas et al, Cellular and Molecular Immunology- Ninth Edition, page 378) As a summary, somehow the self peptide+MHC complex of the DONOR fits in the RECIPIENT TCR to start an immune reaction.
