CURRENT CONCEPTS IN MUCOSAL IMMUNITY - II - ONE SIZE FITS ALL - NONCLASSICAL MHC MOLECULES FULFILL MULTIPLE ROLES IN EPITHELIAL-CELL FUNCTION

The human major histocompatibility complex (MHC) on chromosome 6 encod es three classical class I genes: human leukocyte antigen-A (HLA-A), H LA-B, End HLA-C. These polymorphic genes encode a 43- to 45-kDa cell s urface glycoprotein that, in association with the 12-kDa beta(2)-micro globulin molecule, functions in the presentation of nine amino acid pe ptides to the T cell receptor of CD8-bearing T lymphocytes and killer inhibitory receptors on natural killer cells. In addition to these ubi quitously expressed polymorphic proteins, the human genome also encode s a number of nonclassical MHC class I-like, or class Ib, genes that i n general encode nonpolymorphic molecules involved in a variety of spe cific immunologic functions. Many of these genes, including CD1, the n eonatal Fc receptor for immunoglobulin G, HLA-G, the MHC class I chain -related gene A, and Hfe, are prominently displayed on epithelial cell s, suggesting an important role in epithelial cell biology.