字面上讲是指不同组织共存的能力；严格地讲是指所有移植蛋白的一致性，这是阻止移植和器官排斥的需要。组织相容性的分子基础是修饰几乎所有人类细胞表面的一套移植蛋白。这些蛋白是由位于6号染色体上的一段称为主要组织相溶性复合体基因，MHC编码的。这些蛋白高度多态。例如，它们在不同的人中显示差异。尽管很多人会有一些相同的MHC分子，极少数人有完全相同的MHC分子。微小的差别导致这些蛋白质被移植受体的免疫系统识别为外来的而进行破坏。对成功的移植来说这些蛋白质应该在供体和受体之间相匹配。双胞胎相配的几率最高，接下来是兄弟姐妹。在一般人群中只有10万分之一的比例是MHC匹配的，可以允许移植。

Literally, the ability of different tissues to “get along”; strictly, identity in all of the transplantation proteins, which is a requirement for the prevention of graft or organ rejection. The molecular basis of histocompatibility is a set of transplantation proteins that decorate the surface of nearly all human cells. These proteins are encoded by genes that are grouped on a part of chromosome 6 called the major histocompatibility complex, or MHC. These proteins are highly “polymorphic” i.e., they show variation in different individuals. Although many individuals may share some identical MHC molecules, a very low number share all the MHC molecules. The consequence of these minor differences is that these proteins are recognized by the transplant recipient’s immune system as being foreign, and so are targeted for destruction (since the immune system’s job is to eradicate any foreign proteins or cells that invade the body). For successful transplantation these proteins ideally should be matched between donor and recipient. Twins have the highest rate of match, followed by siblings. In the general population only 1 in 100,000 individuals is sufficiently “MHC matched” to another person to allow transplantation.