Human leukocyte antigen HLA system is a gene complex on 6p21 chromosome encoding the major histocompatibility complex proteins MHC. MHC is a set of cell surface Proteins . MHC molecules bind to antigens derived from pathogens and display them on cell surfaces so appropriate t-cells can attack the cell. So, HLA guides immune system.
They are highly polymorphic and create diverseproteins and unique for persons. Immune system uses these proteins to differentiate between on cells and foreign cells.
in organ transplantation HLA matching is done to compare the donor and recipient. doctors match 8 to 10 markers to find the closest match.
Matching start from family to outsiders. But even after matching the immune system may attack the transplanted organ - organ rejection.
Rejection may be hyperacute immediate. Acute within weeks or chronic in years. So immunosuppressants are used.
Antibody OKT3 used in graft rejection.
Stem cells and by artificial organs will reduce such complexities.