ABSTRACT

The region of the main histocompatibility complex (MHC) that codes for the human Ia-like antigens has been discovered relatively late and has presented a tremendous challenge. The Ia antigens have been difficult because of their limited expression in only certain kinds of cells, and their preponderant manifestation in functional and cellular assays, rather than being ubiquituous targets like the other common transplantation antigens. The allospecificities defined by primary Mixed leukocyte cultures (MLC) with homozygous typing cells (HTC) receive the name of HLA-D antigens. The importance of the Ia-like antigens stems from their role in the genetic control of the immune response, their associations with a variety of diseases, and their probable role in organ transplantation. The close relationship between HLA-D and DR was an impressive finding when the DR antigens were first discovered. The majority of human T cells circulating in peripheral blood do not have detectable la-like antigens.