The HLA System: Biological Function and Association with Disease

Abstract

The HLA system is the major histocompatibility complex (MHC) of man and controls transplantation antigens, various immune responses, certain complement components, and the susceptibility to a variety of diseases. More specifically, the system codes for three sets of characters: the class I-III molecules. The Class I molecules are cell surface molecules carrying the HLA-A, B, and C antigens and present on all nucleated cells and on platelets. The Class II molecules are also cell surface molecules; they carry the HLA-DR, DQ, and DP antigens but are only present on some cell types, macrophages and B-lymphocytes in particular. The Class II molecules are properdin factor Bf of the alternative and factor 2 (C2) and 4 (C4) of the classical complement activation pathway. All Class I, II and III molecules are genetically highly polymorphic: there is a large number of different HLA types in the population, but all HLA factors show pronounced linkage disequilibrium with at least one other factor controlled by genes at a nearby HLA locus.