The HLA-A, -B, and -C antigens are glycoproteins encoded by closely linked genes of the major histocompatibility complex (MHC) located on the short arm of chromosome 6 at 6p21.3 spanning a distance of approximately 3 × 106 bp. These genes are inherited en bloc as a haplotype and are codominantly expressed. Linkage disequilibrium is one of the main features of the HLA system. This is a phenomenon in which the observed frequency of alleles of different loci occurring together is greater in a population than expected by random chance, as calculated from the phenotype frequencies of the individual alleles. The definitive serological test to determine whether a patient is sensitized to antigens present on the lymphocytes of a particular donor is to perform a lymphocytotoxic cross-match. This will not indicate whether positivity is due to HLA or non-HLA antibodies, but the use of dithiothreitol will determine if an IgM or IgG antibody is responsible.