Immune responses to the alpha-Gal xenoantigen

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The successful treatment of many conditions in which the relevant organ has failed completely and irreversibly is to replace the organ with a new one i.e. to perform a transplant. It is well known that there are far fewer organs available for transplantation than the number needed. This means that for those conditions where a supportive treatment is available, eg. the artificial kidney, patients must be maintained by that method, however for other organs such as hearts, lungs and livers, there is no mechanical substitute. If these patients do not receive a transplant, they die. A solution to this problem is to use organs from animals. This is called xenotransplantation. The pig is the most suitable donor, however despite the many similarities to humans which make pigs suitable, there are many differences which are still to be overcome before we can use pig xenotransplants clinically. These differences are at a very fine molecular level and prevent the normal integration of the pig organ into the human recipient. It is well established that the single most important difference which causes this rejection is a sugar molecule like a blood group which pigs have but humans do not. This is called Gal. This is also present in most animal species and we have completely removed it from a strain of mice by genetic modification. Unfortunately it has not been possible to make this change in pigs. However, the genetically modified mice give us a test model to determine precisely how much and by what mechanisms the Gal antigen is responsible for the rejection process.