The histocompatibility antigens or HLA system, are proteic molecular which are localized on the nucleated cell membranes. The system is located on the short arm or chromosome 6 where types of molecular called class I, II and III molecules are coded. Class I genes code for antigens HLA-A-B-C-E-F and G. These genes are made up of one polymorphic glycoprotein heavy chain and one light beta 2 macroglobulin chain. Class II genes are heterodimers composed of a common alpha chain and a polymorphic beta chain. Five well defined subregions have been described: HLA-DR, DQ, DP, DO and DN. The class III or non-HLA products Include complement components C2, Bf, C4A and C4B, 21 hydroxylase A and B, the dipeptide repeat RD gene, the major heat shock protein HSP70, the B-associated transcripts BAT and the alpha and beta TNF (Tumor Necrosis Factor). Because of the system Importance, the laboratory techniques and the molecular biology methods used its study are described: Polymerase Chain Reaction (PCR) and restriction fragment length polymorphism (RFLP).