The HLA class II genes and products: the HLA-D region

In the late 1960s it became clear that certain anomalous proliferative responses in mixed leukocyte cultures (MLC) (1,2) involving cells identical for the HLA A, B and C antigens were a result of disparity for what is now referred to as the HLAD-region(3-5). Later came thefirst evidence that products of the HLA-D region could be defined serologically(6) and a distinction emerged between epitopes recognized on D region products by antibodies (Ia) and those recognized by T lymphocytes (LD). HLA-A, Band C antigens are now known as class I MHC products and D antigens as class II products. Today our understanding of the D region is based not only on serological and T-lymphocyte responses to class II products but also on protein and DNA studies. Here Fritz Bach summarizes this understanding, with emphasis on the T-lymphocyte defined LD system.