A regu­la­ti­on sys­tem in the body important to the immu­ne sys­tem. HLA typ­ing is extre­me­ly important in allo­gen­eic trans­plan­ta­ti­on. The grea­ter the simi­la­ri­ty bet­ween the HLA sys­tems of the donor and pati­ent, the grea­ter the chan­ces of suc­cess­ful transplantation.