**2. HLA antigens**

HLA molecules are membrane bound glycoproteins that bind processed antigenic peptides and present them to T cells. The essential role of the HLA antigens lies in the control of selfrecognition and thus defence against microorganisms. Based on the structure of the antigens produced and their function, there are two classes of HLA antigens, HLA Class I and Class II.

© 2013 Bose et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The overall size of the MHC is approximately 3.5 million base pairs. Within this is the HLA Class I genes and the Class II genes each spread over approximately one third of this length. The remaining section, sometimes known as Class III, contains loci responsible for comple‐ ment, hormones, intracellular peptide processing and other development characteristics [1]. Thus the Class III region is not actually a part of the HLA complex, but is located within the HLA region, because its components are either related to the functions of HLA antigens or are under similar control mechanisms to the HLA antigens.

setting of transplantation. HLA Class II molecules consist of two chains each encoded by genes in the "HLA Complex" on Chromosome 6 [3]. The T Cells, which link to the HLA Class II molecules, are Helper (CD4) T cells. This role of HLA Class II, in initiating a general immune response, is the rationale for their limited expression on "immunologically active" cells (B

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A new HLA nomenclature was introduced in April 2010, replacing a system which had been in use since the 1990's. The main drive for the change was that the old system could no longer accommodate the increasing number of HLA alleles that were being described. There are currently over 5,700 alleles described across all the classical and non classical HLA loci.

The old system was based on assigning significance to pairs of digits in the allele nomenclature (Fig 1). For example in the allele HLA-A\*02010102L, the designation 'HLA' identifies the allele as a HLA allele. The dash (-) separates the HLA designation from the gene, in this case the 'A' gene. The '\*' is a separator. Of the actual allele name, the first two digits (02010102L) represents the allele group and in most instances, was synonymous with the Serological type (A2 in this case). The third and fourth digits (02010102L) identified the specific allele. All alleles whose no‐ menclature differed in these first four positions (02010102L) must code for proteins with differ‐ ent sequences. Alleles whose nomenclature differed in the fifth and sixth position (02010102L) code for proteins with silent mutations within the coding sequences. A sequence which dif‐ fered by mutations in the introns or in the untranslated regions flanking the 3' and 5' ends of the exons were identified by different digits in the seventh and eighth positions (02010102L). In ad‐ dition, a number of suffixes were used to identify sequences that were null, i.e. not expressed (N), those that had low expression (L), those that were secreted (S), those found only in the cy‐ toplasm (C), those with questionable expression and those with aberrant expression (A).

lymphocytes, macrophages, etc.) and not on all tissues [4].

**2.3. Recent changes to HLA nomenclature**

**Figure 1.** Old HLA nomenclature
