Abstract
Human leukocyte antigen (HLA) class II molecules, encoded within the human major histocompatibility complex, play a central role in the immune response by presenting peptide antigens to helper (T4+) lymphocytes. One striking feature is their extraordinary variability between individuals in the population. These molecules, which are expressed on the surface of antigen presenting cells such as monocytes, B cells, and dendritic cells, are the products of the immune response genes originally discovered in the mouse and guinea pig (52). It is now apparent that allelic polymorphisms of class II genes are directly involved in regulating patterns of immune responsiveness in mammalian organisms (6, 74). This realization has provoked a major effort to define the nature and extent of class II polymorphism in the human population. Now almost completed, this endeavor has provided considerable insight into the complexity of the class II gene family and has stimulated new approaches to understanding the relationship between HLA polymorphism and autoimmune disease.