Biology of Disease

individuals having the same ‘set’ of HLA genes is very small, making the pos-

sibility of finding ‘good’ transplant matches, other than identical twins, low.

In the HLA complex, the Class II region is located nearest to the centromere,

which contains the DP, DQ and DR loci. Within each of these loci there are

genes encoding the @ and A chains of the class II molecules. The situation is

rather more complex than for Class I because each locus may contain more

than one gene that encodes the @ and A chains. For example, the HLA-DR

region contains three or four genes for the HLA-A chain. All of the A gene

products may be expressed in a single cell, making the degree of variation

much higher than that of class I proteins. Like Class I, the Class II region also

displays a high degree of polymorphism (Table 6.14).

HLA Typing

Human Leukocyte Antigen typing is the process whereby the HLA antigens

present on the cells of the recipient and the potential donor are determined.

By determining the HLA types of potential transplant recipients, and storing

these details on computer databases it is possible to match the donor organ,

when one becomes available, to the most appropriate recipient. The degree

Class II Class III Class I

DP DQ DR BCE A

HLA Chromosome 6

Figure 6.14Schematic to show the structure of the HLA system on chromosome six.

Gene Number of alleles Number of proteins

HLA-A 372 348

HLA-B 661 580

HLA-C 190 153

Table 6.13Genes and alleles of Class I genes of the HLA complex

Gene Codes for Number of alleles

HLA-DRB Bchain of HLA-DR 249

HLA-DQA1 Achain of HLA-DQ 20

HLA-DQB1 Bchain of HLA-DQ 36

HLA-DPA1 Achain of HLA-DP 13

HLA–DPB1 Bchain of HLA-DP 82

Table 6.14Alleles of the Class II HLA genes

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