An animal will generally accept an organ transplant from itself, but will reject a transplant from other animals, even if the donor animal is of the same species. Organ rejection is primarily a consequence of the interactions between the immune system of the transplant recipient and the histocompatibility antigens present on the transplanted cells. Clinical laboratories play an important role in the histocompatibility testing for solid organ as well as hematopoietic stem cell and bone marrow transplantation (BMT). This chapter provides a brief background to some of the issues and techniques involved in histocompatibility testing related to transplantation. Other applications of histocompatibility testing such as in the characterization of disease states (eg, HLA-B27 in ankylosing spondylitis) or before initiation of some drug therapy (eg, HLA-B*57:01 is a risk factor for hypersensitivity to abacavir) use similar techniques and are not discussed further.

The histocompatibility antigens that are the primary stimulus in graft rejection are encoded by a complex of closely linked genes called the major histocompatibility complex (MHC). In mice, these genes are located on the H2 region of chromosome 17. In humans, the analogous MHC region is located in a 4000-kb region on the short arm of chromosome 6 and encodes for the HLA system (Figure 4–1).

The HLA class I region encodes for certain glycoprotein molecules that are present on all nucleated cells. The main function of the HLA class I molecules is to bind to fragments resulting from the breakdown of intracellular pathogens, such as viruses. The HLA molecule and the bound peptide are then presented on the cell surface so that an immune response can be initiated against the pathogen. Class I molecules consist of 2 noncovalently linked chains. A gene in the MHC encodes the heavy chain, and a gene outside the MHC on chromosome 15 encodes the β2-microglobulin light chain. In humans, there are 3 important class I genes known as HLA-A, -B, and -C. These genes are highly polymorphic. Polymorphism refers to multiple variations of a single genetic locus and its gene product. Each variation of the gene is called an allele. The HLA-A, -B, and -C genes encode for over 100 gene products that can be defined by serology—that is, by matching antibodies against the different HLA antigens (Table 4–1). However, the antigens identified by serotyping are far outnumbered by the alleles that are recognized by sequencing the gene. This is because not all gene polymorphisms result in distinct antibody specificities, although they may stimulate a T-cell immune response.

The class II region encodes for the α and β chains that make up the HLA-DR, -DQ, and -DP molecules (Figure 4–1). These HLA class II molecules have a more restricted expression than the class I molecules. They are found mainly on B lymphocytes, dendritic cells, monocytes, activated T cells, and some endothelial cells. Antigen-presenting cells such as macrophages can phagocytose and engulf bacteria and other parasites via endocytosis. Once these pathogens gain entry to the cell, they can be broken down by proteases into peptides that are able to bind to the class II molecules. These peptides are then presented on the cell surface, and an immune response is initiated. The class III region contains approximately 40 genes that do not encode HLA molecules, but encode certain complement components and numerous other proteins not involved in antigen presentation.

The HLA genes are linked together—in what is called a haplotype—on the chromosome and inherited en bloc. Each individual inherits 1 haplotype from each parent, and the 2 haplotypes represent the HLA genotype. The alleles from both of these haplotypes are expressed on an individual’s cells. This is referred to as a codominant expression of the gene. Therefore, even though there are multiple HLA genes, usually only 4 genotypes are possible in the offspring. There is a 25% chance of any 2 siblings being HLA identical and a 50% chance of the siblings sharing a haplotype.

HLA antigens and alleles are named by the World Health Organization Nomenclature Committee for Factors of the HLA System. New alleles are named on an ongoing basis as they are identified, and the number of HLA alleles has grown rapidly, with over 8000 currently listed. The DNA sequences of all recognized HLA alleles are maintained in the IMGT/HLA Database and are available online (http://www.ebi.ac.uk/ipd/imgt/hla/). Each HLA allele name follows a strict format defined by the Nomenclature Committee (Figure 4–2).