HLA typingBMJ 2001; 322 doi: https://doi.org/10.1136/bmj.322.7280.218 (Published 27 January 2001) Cite this as: BMJ 2001;322:218
Human leucocyte antigens (HLAs) are cell surface molecules found on all nucleated cells. Each individual has a unique set of these antigens, half inherited from each parent, and their typing becomes important before organ transplantation. Typing is also used to identify markers for specific diseases, such as HLA B27, which is known to be closely associated with conditions such as ankylosing spondylitis.
Two main classes of HLA antigens are recognised: HLA class I and HLA class II. HLA class I antigens (A, B, and C in humans) render each cell recognisable as “self,” whereas HLA class II antigens (DR, DP, and DQ in humans) stimulate the immune system.1 Both have been implicated in the rejection of transplanted organs.
Three main processes are used to perform HLA typing. The first is the more conventional serological cytotoxicity method where tiny samples of lymphocytes (taken from from blood or spleen) are added to Terasaki plates. These plates hold individual wells that contain different specific antibodies (from either maternal sera or manufactured monoclonal antibodies). The best cells for class II typing are B lymphocytes, and class I typing can be performed with the remaining leucocytes. Magnetic beads are used to purify the required cells from blood or spleen.
If the HLA antigen and specific antibody bind, and complement is added, the cells in that well will be killed. The pattern of wells showing this cell death allows the deduction of which combination of HLA antigens were present on the original tissue cells.
Another potential method used for HLA typing is flow cytometry, particularly when looking for specific alleles. Here fresh nucleated leucocytes are added to monoclonal antibodies that are labelled with a molecule that fluoresces. Cells with surface antigens that bind to the antibody become fluorescent. The flow cytometer detects the fluorescent cells by detecting the light emitted from them as they pass through a laser beam. Flow cytometry takes about 30 minutes to complete—the time taken to prepare the cells and then run the machine.
A third process is gaining favour where very detailed typing is required—for example, for precise matching in bone marrow transplantation. This process involves extracting the DNA from cells and amplifying the genes that encode for the HLA peptides using polymerase chain reaction techniques. The genes may be matched with known HLA nucleotide sequences found stored in several gene bank databases, including the IMGT/HLA database.
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