The HLA complex

Peptides encoded within the major histocompatibility complex (MHC) have the ability to provoke vigorous rejection of grafts in transplantations (Roitt, 1994). The complex, also referred to as human leukocyte antigen (HLA) region, can be divided in 3 regions: class I, class II and class III (Figure).

Class I and II molecules are membrane-bound proteins which are recognized by T-cells; class I molecules signal for cytotoxic T-cells, while class II signal T-helpers for B-cells and macrophages. Class III molecules include complement components, heat sock proteins and tumor necrosis factors. The recognition of the class I and II molecules by T-cells mediates the graft rejection response in transplantations. For this reason, histocompatibility testing is mainly focused in these two regions. Class I HLA-A and HLA-B molecules are expressed abundantly and are highly polymorphic.

HLA-DP,-DQ and –DR are also polymorphic. Population studies have shown that matching of the above alleles is associated with lower risk of graft failure; moreover, clinical studies have demonstrated that the use of molecular methods for HLA typing improves the efficiency of haematopoietic cell transplantation (Petersdorf et al., 2003).

 

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