HLA typing in organ transplant part 7

HLA typing and its influence on organ transplantation

The selection of the optimal donor is based on high-resolution HLA typing. The MHC (Major Histocompatibility Complex) contains more than 200 genes which are situated on the short arm of chromosome 6 at 6p21.3 The role of HLA molecules is to present peptides to T cells (both CD4 and CD8 T cells), enabling them to recognize and eliminate "foreign" particles and also to prevent the recognition of "self" as foreign. HLA mismatches may occur at antigenic or allelic level; the first are characterized by amino acid substitutions in both peptide binding and  T cell recognition regions , whereas the latter are characterized by amino acid substitution in the peptide binding region only. When a human transplant is performed, HLA molecules from a donor are recognized by the recipient's  immune system by direct and indirect  methods of  allorecognition triggering an  alloimmune response. Matching of donor and recipient for MHC antigens showed a significant positive effect on graft acceptance. In organ transplantation, the adaptive immunity is the main response exerted to the transplanted tissue, since the main target of the immune response is the MHC molecules expressed on the surface  of donor cells. T cell activation leads to the production of cytokines and chemokines which in turn may recruit components of the innate immunity like natural killer cells or macrophages and complement. In addition to that, defensins and cathelicidin have chemoattractant properties on T lymphocyte.