Fig. 2. The schematic diagram presents the development of T cells in thymus. Early T lymphocyte progenitors in cortex of the thymus are defined as double positive (DP) thymocytes. In the medulla region, DP precursors are acquired to facilitate maturation by MHC molecules. The negative selection procedure is completed by apoptotic cells which failed to eliminate the T cell receptor and to convert to single positive (SP) cells. Although thymus environment of mouse does not fully support HLA expression in T cells, human mimic factors including Notch signal factor DLL4 and chemokines such as Ccl25 and Cxcl12 can contribute expression of HLA molecules. MHC-I can bind with CD4⁺ T cells and MHC-II class has affinity towards CD8⁺ T cells. Matured T cells can provide information of immune response in terms of donor HLA molecules. MHC, major histocompatibility complex.