Formal genetics of the HL-A region

The extreme polymorphism of the HL-A system is due to the presence of two (SD1, SD2) and perhaps three linked polyallelic genes. The distinction of "bridging antibodies" (reacting with several HL-A specificities recognizing separate sites on the HL-A molecule) from the main HL-A determinant as it is demonstrated by absorption/inhibition experiments increases this complexity. The HL-A linkage group is composed of other systems: LD1, LD2, PGM3, ADA (?), P, ME1, IPO-B and possibly a "hay fever gene". No gametic or zygotic selection was found in spite of the presence of HL-A antigens on spermatozoa. Mixed lymphocyte reaction (MLR) is principally governed by LD genes. The main (LD2) gene is probably situated outside the interval SD1, SD2, near SD2. Other LD genes (LD1 inside the interval SD1-SD2 and LD3) are suspected. The presence of an immune response gene (Ir) has not yet been demonstrated although several diseases associated with specific SD2 antigens are known. These different genes (SD1, SD2, LD1, LD2, LD3 and Ir) probably form a functional unit in the allo-immunozation.