Hla Genetic Organization

The HLA (human leukocyte antigen) complex is the name of the human major histocompatibility complex (MHC). It is located on chromosome 6 (6p21.3), covers about 3600 kilobases of DNA, and contains in excess of 120 expressed genes.4 5 Two classes of HLA molecules are central to the control of immune responses: class I (HLA-A, -B, -C) and class II (HLA-DR, -DQ -DP)67 (Figure 93.1). Class I molecules are heterodimorphic glycoproteins whose alpha chain (~45 kD) is encoded by HLA-complex genes (HLA-A*, -B*, C*) and whose beta chain is ^-microglobulin 02-M), encoded on chromosome 15 (l5q21-22.2).8 Class II

molecules are also heterodimeric glycoproteins, but both alpha and beta chains (~28-32 kD) are encoded within the HLA complex (alpha genes HLA-DRA1', -DQA1*, -DPA1* and beta genes HLA-DRB1', -DRB3', -DRB4', -DRB5', -DQB1*, -DPB1 *).

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