ABSTRACT
The human lymphocyte antigen (HLA) system was originally defined as the human histocompatibility complex (MHC) because its products are strong histocompatibility antigens. HLA class I and II products are essential for T-cell activation and for the regulation of the immune response. Moreover, they are immune response gene products because their polymorphism leads to interindividual differences in immune responsiveness. HLA class I molecules are produced by and present on the membrane of basically all nucleated cells. HLA class II molecules are mainly present on cells of the immune system: macrophages and other antigen-presenting cells, B-cells, and activated T-cells. The etiological fraction indicates how much this HLA-associated factor contributes to susceptibility for a given disease at the population level. The problem with this approach is that it involves two unknowns: HLA and autoimmunopathogenesis. Most autoimmune diseases are associated with class II HLA antigens. Another observation of more recent date is the expression of class II molecules on cells which normally do not express these molecules.
