Old game, new players: Linking classical theories to new trends in transplant immunology

Figure 2 Antigen presentation and allorecognition. T cells can recognize alloantigens by three different pathways of allorecognition: (1) Direct pathway involves the recognition of intact donor MHC molecules on the cell surface of donor APCs by recipient T cells; (2) In contrast, in indirect pathway donor MHC molecules are processed and presented as peptides by recipient MHC molecules to recipient T cells; and (3) Semi-direct pathway, in turn, involves the transfer of intact donor MHC molecules to recipient APCs, being presented to recipient T cells. MHC: Major histocompatibility complex; APCs: Antigen-presenting cells.