The HLA linkage group and disease susceptibility.

The HLA linkage group, located on chromosome 6, is composed of serologically defined (SD-) loci (HLA-A, -B and -C) and of a MLC (= mixed lymphocyte culture-) or lymphocyte defined (LD-) locus (HLA-D). Different alleles of these loci are linked together as well as with loci for the synthesis of some complement components and the properdin factor B (= Bf-system) by strong positive linkage disequilibria (gametic association). The indication of a HLA linked genetic control of immunoresponsiveness makes the HLA system one of the biologically most important immunogenetic systems in man. The associations, found between single HLA determinants and diseases, can be explained in different ways, but may contribute to a better understanding of the etiopathogenesis of diseases and of the biological role of the HLA linkage group.