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How do HLA antigens influence organ transplant matching?

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Kidney transplant — transplant British surgeon Andrew Ready flushing a donated kidney during a transplant at Queen Elizabeth Hospital, Birmingham, England, 2006.

Human leukocyte antigens (HLA), which constitute the major histocompatibility complex (MHC) in humans, are critical factors in transplant surgery because they determine an individual's tissue type.12

When an organ is transplanted, the recipient's immune system recognizes foreign HLA antigens as threats, which can trigger graft rejection. These antigens are categorized into two primary groups:32
* Class I antigens: Present on most cells, these serve as the targets for the body's effector rejection response.32
* Class II antigens: Primarily found on specific cells like dendritic cells, these act as the initiators of the rejection reaction.3

To minimize the risk of rejection, medical professionals perform tissue typing to compare the HLA profiles of the donor and recipient. While a perfect match is ideal, it is rare outside of identical twins or close relatives. Consequently, many transplants involve less closely matched tissues, requiring the use of immunosuppressive drugs to prevent the immune system from attacking the donor organ. A final direct crossmatch is typically performed to ensure the recipient does not already possess antibodies that would immediately reject the donor tissue.132