Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Direct and indirect alloantigen recognition are two mechanisms by which T cells may recognize alloantigens and lead to transplant rejection after an organ transplant.Donor tissue macrophages migrate to lymph nodes, stimulate a measurable percentage of recipient T cells. The host T cells in the lymph node recognize either the allograft HLA or an associated bound peptide.In this case, alloreactive T cells are stimulated by donor APCs which express both the allogeneic MHC and costimulatory activity.Host (organ-recipient) macrophages process proteins from the transplanted graft, and trigger a T cell response. Presentation may occur via MHC class I or MHC class II, although class II is usually involved.