HLA class II antigens: structure, function, and expression in immunodeficiencies, autoimmune diseases, and allograft rejection.
The major histocompatibility complex (MHC) genes, coding for the histocompatibility leukocyte antigens (HLAs) are located on the short arm of chromosome 6. The HLA gene products are involved in many immune responses. Class I and class II molecules are composed of two subunits. Class I antigens are expressed on all cell types except red blood cells; class II molecules have a more limited distribution and are found only on certain cell types. A review of our current knowledge of the genes coding for these molecules, their biochemical analysis, and the current investigation into the three-dimensional structure of the glycoproteins will allow us to understand better how this complex system, which has been preserved in animal species throughout the evolutionary process, operates. The biologic significance of HLA class I and class II molecules in the development of an efficient immune system that permits our survival in the sea of microorganisms in which we are immersed is illustrated by the severe and lethal infections that occur when these antigens are absent. Important and interesting variations in class II antigen expression are also observed in certain autoimmune diseases. Furthermore, the role of these antigens in the allogeneic reaction of graft rejection has now been clearly identified. It has become increasingly apparent how important it is to understand the antigenic variation between organs, or even between cells within an organ, and to be able to identify the circumstances that are associated with modulation of antigen expression.