The HLA (human leucocyte antigen) system is a system of genetically determined tissue antigens that can be found on cells of nearly all tissues. HLA antigens play a major role in immunological defence mechanisms, i.e. they enable the immune system to distinguish between endogenous and foreign cells. In the context of transplantation of organs or foreign tissue, this detection system has to be avoided. Therefore it is necessary to find an HLA compatible donor for a particular patient, i.e. a donor whose HLA system is similar to the recipient’s in order to minimise immunological defence reactions.