Having two different classes of HLA molecules with different gene representatives or isotypes (isotypic diversity), and each gene isotype having different alleles (allelic variability) is greatly responsible for the diversity of the HLA system at the population level. At the individual level, being most humans heterozygous for HLA alleles and having their HLA alleles on each parental chromosomes expressed- co-dominantly on the cell surface, increases further their antigen-presenting repertoire. Furthermore, HLA haplotypes contain different numbers and combinations of distinc expressed HLA-DR beta genes, which contributes to further increase the diversityof the peptides to be presented. Finally, the alpha and beta chains of some of the class II HLA isotypes (in particular HLA-DQ and -DP) have the capacity to dimerise between each other. This process is referred to as cis-dimerisation when occurs between expressed products from the same chromosome, and as trans-dimerisation when occurs between different chromosomes. The total number of HLA alleles, null and expressed as protein products, are reported and updated periodically on the IMGT/HLA database. But the total number of existing expressed HLA forms in the entire population and in a given individual might differ depending on how well or not some of the HLA molecules dimerise with each other.