Still more genes in the MHC

The major histocompatibility complex (MHC) encodes three major classes of molecules. Two of these, class I and class II molecules, act as templates to 'present' antigens. Class I molecules, expressed on most cells, are implicated particularly in presentation of antigens to cytotoxic T cells - by cells infected with virus, for example. Class II molecules, encoded by the so-called 'immune response' genes (HLA-D region in man, la in the mouse) are constitutively expressed on certain immunocompetent cells, such as B cells or macrophages, and can be induced on many others with γ-interferon. Class II molecules generally present antigens to helper T cells and can, therefore, influence subsequent production of specific antibodies. The other major class of molecules in the MHC, class III, includes the complement components C2, C4 and factor B(1). The MHC has intrigued a generation of immunologists and geneticists because of its inordinate polymorphism, but in the past four or five years the field has been plundered by molecular biologists armed with recombinant DNA technology. Just how far this line of investigation has got in understanding the MHC was assessed at a recent conference.