<unk><unk><unk>  (HLA) began as a list of<unk>  identified as a result of transplant rejection. The<unk>  were initially identified by categorizing and performing massive statistical analyses on interactions between blood types. This process is based upon the principle of serotypes. HLA are not typical<unk> , like those found on surface of infectious agents. HLAs are alloantigens, they vary from individual to individual as a result of genetic differences. An organ called the thymus is responsible for ensuring that any T-cells that attack self proteins are not allowed to live. In essence, every individual's immune system is tuned to the specific set of HLA and self proteins produced by that individual; where this goes awry is when tissues are transferred to another person. Since individuals almost always have different "banks" of HLAs, the immune system of the recipient recognizes the transplanted tissue as non-self and destroys the foreign tissue, leading to transplant rejection. It was through the realization of this that HLAs were discovered.
History and naming of human leukocyte antigens
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