[55] HLA Antigens in Serum

This chapter explores that human histocompatibility antigens are sets of highly polymorphic glycoproteins encoded within the MHC region that play a critical role in cooperation among immunocompetent cells. HLA-A, B, C antigens are expressed on nucleated cells while Ia antigens have a restricted tissue distribution, as they are mainly expressed on cells associated with immune functions. The chapter reviews that the allospecificity HLA-A9 is found in serum in significantly greater amounts than any other allospecificity. It discusses that HLA-A, B antigens in serum exist as lipid-glycoprotein complexes, probably as a result of shedding of these integral membrane proteins from the cell surface with their associated boundary lipid intact. The behavior of these serum antigens upon ultracentrifugation at different densities indicates that the major histocompatibility complex (MHC) antigens coisolate with the high density lipoproteins. The molecular structure of HLA-A, B antigens derived from the serum is similar to that of antigens isolated from lymphoid cells. It reviews that the size of the heavy chain of serum HLA-A, B antigens is similar to that of HLA-A, B antigens isolated from cells with detergents and larger than that of HLA-A, B antigens solubilized with papain. The chapter concludes that detection of specific HLA antigenic activity by serum typing offers potential advantages over cellular typing in several circumstances, such as when patient lymphocytes are not readily available in retrospective studies and in studies of diseases with a fatal outcome.