The T cell response to major histocompatibility complex (MHC) alloantigens occurs via two
main pathways. The direct pathway involves the recognition of intact allogeneic MHC:
peptide complexes on donor cells and provokes uniquely high frequencies of responsive T
cells. The indirect response results from alloantigens being processed like any other protein
antigen and presented as peptide by autologous antigen-presenting cells. The frequencies
of T cells with indirect allospecificity are orders of magnitude lower and comparable to other …